Hey all, Im buying a trek 820 this weekend, and I am 5'11.
IO am going to be test riding a few sizes, but I just wanted to know what sizes would probably fit me well..The ones availible are.. 13, 16, 18, 19.5, 21";
As I've found out recently, frame sizes vary greatly from one manufacturer to another. I'm 5'7" and have been riding a 17.5" Gary Fisher for 6 years. I recently bought a 16" Kona and it turned out to be the same size in terms of length and standover as my fiancee's 14" Orange. I've exchanged it for an 18" and even that is smaller than my Gary Fisher, again in terms of length and standover.
The best thing to do is get on the bikes and see which feels best for you. Some people like the bike to be small and chuckable and others like a long stretched out feel. I fall somewhere in between and like to ride a long bike with a short stem.