it sort of depends on the bike, some bike's ride smaller or larger, depending on the geometry and how they are set up. For example, i'm 5'5", i would normally ride a 16" frame or so, but on the new Rocky Mountains, i fit quite comfortably on a much larger frame, which made no sense at all, but there i was and i was comfy. it also depends on if you have long legs and a short torso, or short legs and a long torso. at 5'6" you need a "medium" bike, i would say around the 16-18 range.