One that allows you to comfortably reach the pedals, brakes, and water-bottle cages. If the seatpost is barely showing and you feel stretched out, the bike is too large. If you need to extend the seatpost near or past its minimum insertion (which is marked on the post) or think you need a stem longer than 110mm, you could probably go larger. We always recommend trying a bike before you buy it or visiting a bike shop for a professional fitting.