It somewhat depend upon the brand. I'm 5'-10" as well, and according to some brands I should be medium, and others I should be large. I'm a tweener. That's where riding various bikes is important to figure out which is more comfortable for you. By the books most would say medium for me, but I find I am more comfortable on a large. Some people with the same height and inseam will find the medium more comfortable. No way to know, except to go climb on as many as you can.

QT