Anonymous wrote:0735 here again:
This is basically what you need to know to size a bike:
-- Have him stand over the bike, feet flat on the ground. There should be about 1" of clearance between his crotch and the cross bar. He should NOT be a tippy-toes.
-- When he sits in the saddle and puts his feet on the pedals, the leg that's fully descended should have only the slightest bend at the knee. Similarly, the other leg at the upswing of the pedal should be comfortable and not in his chin or something.
-- You should be able to adjust the saddle to get him to a correct height (although if you're looking at cheap bikes, maybe not).
Here's a pretty good overview:
http://www.peterwhitecycles.com/fitting.htm
One other thing, I presume you know how to fit a helmet? They need to fit square on the head, protecting the frontal lobe. They don't get worn back on the head.