Craig, my kids are running 41-16 gears-they are 6 and 8. Colin (8) can pull 42-16 at HC, but he says he likes the 41. The 8 year old is on 150mm cranks, I just ordered a set of 140’s for the 6yr old to try out. Dotek cranks are only like $30 from Dan’s or J&R. They were both on 155’s until a few weeks ago. I am certainly no expert at this either, alot of it has been other people’s advice and some research and alot of trial and error! If you bring him out to HC he can certainly try out my kids bikes. Kyle has a mini, Colin has an expert which he is finally growing into. Both Haros, FWIW.