We definitely don't want you to be on the wrong size bike! You should look cool and be comfortable riding your Koben, so we've been working with our frame manufacturer to offer XS and XL frames as well.
UPDATE (5/22): It isn't looking likely that we can offer additional frame sizes given the low number of bikes backed. I think folks who need XS or XL frames will have to wait until we are in full production next year.
The S, M, L should fit most people between 5'3" and 6'3". We wish we could put in a bigger order and offer five sizes instead of three.