I used to get mine at Underworks, but the sizing was always off--I wound up needing a size smaller than their charts indicated. But I've also seen posts from a guy or two who said that this was no longer a problem. So you might be able to simply use the sizing instructions on the site (or whatever site you go to). To be sure, you might want to do an advanced search of Susan's Place (I never get good results with the regular search) and just look for "underworks" or "underworks" and "sizing" or "size" and see what guys have been saying for the last couple of years. Or just sit back and wait for responses in your own thread--you just got one that comments on sizing.
I bought only one binder the first time because I wasn't sure of the size. You can return them if they are essentially unused (if you don't mind losing what you paid for shipping). The one I bought started out just okay, but it stretched out a little and I was losing a lot of weight. So the next time I ordered, I went smaller and got two or three so that I could rotate them. Not too much after that, I was doubling them to get more compression, but that was probably because of my skin allergies--I had to buy the cotton-rich Cotton Concealers that aren't quite so powerful as the other binders. If you get one of the killer smasher models, you shouldn't need to double up.