Programmers come in two main sizes: too skinny, and too fat. If your engineering team is mainly "seasoned pros" you want to avoid american apparel altogether. If it's fresh grads you have a few years worth of size medium before 14 hour days and free soda catches up with everyone.

It's different if you are buying shirts as schwag. I once worked for a startup where the t-shirts were more popular than the product and the most important sizes were mens and womens size medium.