I know this is an older thread, but,
Kapusta, I'd like to know the specific measurements you used to decide small was a good fit for you. I'm also about 5'10" & looking into a Surly Wednesday, but I figured medium would probably be the best bet (or would have sized up to a large rather than down to a small). I've been riding a 19.5" '84 Stumpjumper Sport for summer trails & although I know it is not an apples to apples comparison, it is a good fit.