Well it's really just trail and error, it took me forever to find one that fits.
However, being an ex football player, you should be warned against wearing a cup. There was a guy on my All-Ontario team a few years ago, and he got tackled and the cup shifted, and split one of his testicle. It required surgery and shit, I'm not exactly sure the end result. But if you do chose to wear a jock in football, make sure it's ultra tight fitting