The Golden Rule: Get measured, head to toe. Know your shoulder width, neck size, chest size, true waist size, torso length, the size where you actually wear your pants, inseam, outseam, shoe size, the works. We are notoriously difficult to fit, and we make it harder on ourselves when we don’t know exactly the sizes we need things to be.

Six axioms of big guy fashion:

1. Looking good as a big guy is harder than looking good as a skinny guy, but it is far from impossible. It just requires patience and a vow to never shop at Casual Male XL.
2. Go ahead and resign yourself to the fact that you will be doing most of your clothes shopping online. Always be sure to check the return policy before you buy so you don’t get stuck with clothes that don’t fit.
3. Of the major brick and mortar department stores, Kohl’s caters best to the gentleman of stature in terms of sizing, you just have to shop there with a discerning eye, as they carry a lot of brotastic doucherags.
4. Worry about fit first and following trends second. You aren’t going to look like Daniel Craig in a slim fitting Savile Row suit, but you can look good.
5. Don’t wait until you lose weight to buy good looking, well fitting clothes, and don’t buy things that are too small in anticipation of losing weight. The confidence gained by dressing well goes a long way to building up low self esteem, and building up your self esteem provides greater motivation to lose weight. Self esteem is gained by doing estimable things. Remember that expensive items like suits and sport coats can always be taken in (within reason, you obviously can’t turn a size 50 jacket in to a size 40 jacket) as you lose weight.
6. Bagginess kills. You don’t want things to be so tight that you are straining buttons and walking around with a perma-wedgie, but you don’t want things to be flowy and loose. Clothes that are too big just make you look heavier.

Jackets/Suits:

If you can afford it, just go ahead and get made to measure jackets or suits. It is just easier that way. With that said, here are a couple of things to consider when buying off the rack: First, if it doesn’t fit in the shoulders, it doesn’t fit. Wearing a jacket with shoulders that are too big is an one way ticket to schlubville. Once you have found a jacket that fits in the shoulders, button it- just the top button for a two button jacket and just the middle button for a three button jacket. Don’t wear a four button jacket. If you can’t button it at all, try a different jacket, since it can’t be let out that much. If you can almost but not quite button it, you might be able to have it let out enough, but it is a gamble. If you can button it but it is tight, you can probably have it let out enough for it to fit properly. Two vent jackets look better on big guys than one vent jackets. Center vents tend to gap open more readily than side vents do. Also, if you are at a function in the summer where you have to wear a suit or jacket, don’t take it off, even if the host says that Gentlemen may remove their coats. NEVER take off your coat if the host is wearing his. Obviously you don’t want to give yourself heat stroke, but sweat stains aren’t a good look.

Formalwear:

Go ahead and start saving up now for a tux if you think there is a slim chance that you might ever need to wear one. There has never been a rental tux that looked good on a big guy. Hence, you pretty much have to buy one. Remember, a cheap tux that you own looks far better than an expensive one that you rented.

Dress Shirts:

For a button down shirt that you are going to wear with a suit or sport coat, your fit priorities, in order, are as follows: First, make sure it fits in the collar. It shouldn’t be tight, but it shouldn’t be too loose either. If it is making your neck form a muffin top over your collar, it is too tight. If you can fit more than a thumb between the collar and your neck, it is too loose. Second, when trying on the shirt, tuck it in and sit down. If it strains the buttons around your gut, try a different brand or different size. You want the tightest fit you can find that won’t unduly strain the buttons when you sit. Button downs are an area where the trend towards slim fits tends to violate the laws of fat guy physics- our guts stick out farther when we are sitting than when we are standing. Sleeve length isn’t as important as these first two things when you are wearing the shirt under a jacket, since the jacket sleeve hides the blousing. Note that it is possible to find button down shirts that tick all of the boxes and fit well off the rack, but it does take some looking. If you exhaust all of your options, you can always have a shirt tailored or order one made to measure. If you need a 2x or smaller, Brooks Brothers shirts tend to be a great fit for big guys, as they are plenty loose around the middle. Look to see if there is a Brooks Brothers outlet in your area.

Pants:

First, never, ever, ever, ever wear pants with pleats. Ever. They will only make you look fatter. Getting the length of your pants correct is the most important thing other than making sure that they are pleat free. It is crucial that your pants have little to no break. Cuffs are dependent on the pants, your build, and your height. If you have longer legs and you are wearing a pair of wool flannel or tweed trousers, a cuff can look good. If you are short and you are wearing khaki chinos, avoid cuffs like the plague. Another thing to avoid like the plague are cargo pants. Cargo pants on big guys are almost as bad as pants with pleats.

Shorts:

Don’t. Just don’t. I don’t care if it is 100 degrees and humid. Get some lightweight chinos instead. Big guys don’t look good in shorts. We look like a stereotype of a Scoutmaster or a tourist in shorts. If you are going to the beach, and you aren’t going swimming, the best look you can go with is the aforementioned lightweight chinos rolled up a bit and a white OCBD, untucked, with the sleeves rolled up.

Jeans:

This is a minefield. The best jeans I have personally found are the Gap 1969 standard fit jeans (resin rinse). They are available in waist sizes up to 44, with 28 inch inseams running to 40 (these are the only 40/28 jeans on the market that I know of) and 34 inch inseams running to 42. 44s are available in 30 and 32.

Casual Button Downs:

Casual button downs: Don’t wear anything that will make you look like a bowler, and don’t wear hawaiian shirts. For a casual shirt that you won’t be wearing a tie with, the fit around the gut is more important than the fit at the collar, since you probably won’t be buttoning the top button.

T-Shirts:

Kohl’s carries tees in (n)XB sizes (the B stands for Big). These shirts have the torso length of an (n)XT (tall) tee with the sleeve length of a standard tee. The bane of the big guy’s existence, and the fastest way to look terrible, is to have your gut hanging out of the bottom of your t-shirt. These solve that problem. Look around, other companies may do the same thing, but I don’t know of any offhand. Never wear t-shirts with any sort of branding, logo, image, or pithy quote on them. You will automatically look like ‘that guy.’

Accessories:

Belts: Never, ever, ever, ever wear a skinny belt, even if skinny belts for some unforeseeable reason come back in to fashion. You want a belt that fills your belt loops, and if your belt doesn’t quite fill your belt loops, make sure that the belt is up high in the tops of the loops and not down in the bottoms of the loops. If the belt is in the bottom of the loops, the top of the waistband of the pants is likely to splay out and look crappy. Also, make sure that your leather belts are made from one solid piece of leather. Avoid belts that are multiple pieces of leather that have been glued together, for the plies will split apart on you.

Suspenders: Suspenders are often better for keeping your pants up than belts. However, unless you are bald and have a handlebar mustache, never wear suspenders unless you are wearing something over them, like a sweater, vest, or jacket. Also, NEVER wear suspenders with a T-shirt.

Socks: Look for tall socks. They will stay up better than normal length socks.

Ties: Look for extra long ties. You need a longer tie because the line from your collar to your belt is not a straight one, and you need to cover the extra real estate. Sadly, the most commonly found extra long ties are made by Donald Trump. However, he does make nice ties, assuming you can find one of the patterns to your liking. If you absolutely can’t find an extra long tie to your liking, and you have to wear a standard length one tied with the skinny end very short, use a small safety pin to pin the skinny end to the back of the wide end, without having the safety pin go through the front of the tie. You can then use a tie bar to hold the tie down and not reveal your cheat. As far as knots go, the four in hand is too skinny for big guys, but the full windsor takes up too much tie length. The half windsor is the best compromise I have found. If you insist on a full windsor, you pretty much have to have an extra long tie. For bow ties, you need a larger bow than skinny guys. Wearing a standard or small sized bow tie makes your head look massive.

Scarves: Look for longer scarves. Not Doctor Who long, but longer than the normal short ones you find in stores. You need a longer scarf for the same reason you need a longer tie.

Hats: No. You can’t pull it off. Don’t even try. You can make an exception if you are on a boat or on safari. Get a good pair of sunglasses instead. If you aren’t sure which sunglasses to get, get a pair of Wayfarers. Wayfarers look good on almost anyone.

Wide Shoes (thanks to /u/hcsteve ):

The following MFA favorites are available in wide sizes:

Allen Edmonds Strand (E and EEE), along with most of AE's other shoes Red Wing Iron Ranger (EE) LL Bean Boots (EE) Sperry A/O (W & XW) Bass Brockton (EE) 

From jdbee’s guide (thanks Syeknom):

Bass Logan penny loafer (up to 5E!) LL Bean Blucher Moc (EE) Sperry Stripers plimsoll (EE) New Balance 574 (EE) Custom shoes from Quoddy, Russell, or Rancourt 

And of course, the copy/paste killed all of the links. They can be found below in hcsteve’s post below. Throw him an upvote as you pass by.

Miscellany:

Never wear workout clothes in public. Ever. If you are going to the gym, carry your workout clothes in a bag and change at the gym. Workout clothes on big guys are a bad look. Along the same lines, unless you are a sumo wrestler or a defensive lineman, you aren’t an athlete, so don’t smell like an athlete. Shops like Lush and Kiehls offer a good selection of personal grooming products that will leave you with a unique and distinctive smell. When it comes to attracting those that we are attracted to, we are at a natural disadvantage due to our size, and smelling good goes a long way to ameliorating that disadvantage. I get compliments nearly every day from women on my smell, even as a smoker. It helps a lot.