I'm 5'6" and ride 50 or 52, depending on the bike. 50 errs a tiny bit small, while 52 is just a tiny bit big.

Keep in mind, when standover is referred to, it's not from where your, um, dangly bits first even hint at brushing the top tube...when I stand over my bikes my underside is in fairly good contact with the top tube, but I can pull up FIMRLY and get the front wheel greater than an inch above the ground. Pulling not so firmly gets me an inch+ easily.