I am 5' 10'' and I have a 54cm bike. Measure your inseam in cm and multiply that by 0.65. This will give you a rough estimate on what size bike you should get. Ultimately it comes down to how it feels, however, when you buy online you don't get that option.
Try this website
http://www.ehow.com/how_14104_determ...ect-frame.html