Your best bet is to visit a bike shop that can help you with fit. Failing that, we posted a basic fit chart here
to help you establish a rule of thumb when looking at Surly bike sizing. Additionally we wrote about the whole thing in this article found elsewhere on our website:
You might also want to read this from the late bike tech guru Sheldon Brown:
Or for the laziest of you:
(Make sure you are referencing your "cycling inseam" and not your pants inseam: Stand with your back against a wall, your bare feet 6" apart on a hard floor, looking straight ahead. Place a book or carpenter's square between your legs with one edge against the wall, and pull it up firmly into your crotch. Have a helper measure from the top edge of the book to the floor. (You can convert inches to centimeters by multiplying inches by 2.54.) Repeat two or three times, for consistency, and average the results to get your inseam length.