Hi David,
The Halfbike 2's sizes depend on your height: If you're between 5'3'' / 160cm to 6'2'' /186 cm you should get a size M. If you're between 6'2'' / 187 to 6'5'' /195 you should get a size L
The Halfbike 3's sizes also depend on on your height, but the system will choose the appropriate size after you enter your height. We may occasionally ask for one more measurement.
Let me know if I can help with anything else.