I actually checked the one Speedo brand suit I have, and when compared against several other brand suits, the Speedo is indeed larger around the waist for the same size. So it seems that Speedo brand's sizing guide is correct. (See URL below)
You need to measure your waist, and then downsize by two to three inches. The amount depends on the size. The larger your waist, the less (2") you downsize. But with Tyr, for example, I do not need to downsize.
So I realize trying it on is the best thing, but if you want to buy by mail order, I want to share the lesson you can avoid mistakes. As Byron, louis and others said, it is really difficult. But at least for Speedo brand, I think I figured it out.
http://www.speedousa.com/product/siz...oryId=15682446