What size do I need if I'm in-between sizes? If you're in-between two sizes, both the smaller and larger frame would be suitable, the key measurement is "Reach", which is basically how long the bike feels when you're in the riding position. Your reach is influenced by your upper body length (also known as your "trunk length or sitting height). If your trunk length is longer than average, then we'd recommend the larger of the two sizes, or if your trunk length is shorter than average, then go for the smaller of the two sizes.