I'm 5'10" tall and just bought a 20" Trek 7.2 fitness bike. Does anyone 5'10" tall own a 20" ? Is this the correct size or should I have gotten the 22.5".
Does it feel OK to you? That's what's important. Trek calls that size 7.2 a "medium", and you're about average in height, so it's probably at least close, unless your proportions are decidedly non-average.
What really matters most for bikes of that type is whether you can get the seat and the handlebars where you want them. If you can, it fits... but another size might fit too. It doesn't really matter, because what you mostly get with the smaller size is just the same bike with a shorter seatpost. So, you end up with more seatpost length exposed, that's all. The one thing you have to look at is whether you feel good with the handlebars where they are: not too cramped up, and not too stretched out either.