What are your thoughts on kid size to sled ratio. My little brother is 9 and is a shrimp (80 pounds 4' 2"). He rode a 120 until last year when i upgraded him to a 300 freestyle which fits him ok, but maybe a bit big. What size have you moved kids to a pro rmk 600 size sled? He will be on this 300 a minimum of another year just learning the basics in the Black Hills in South Dakota as well as at home. Definitely not ready size wise or experience wise this year, but also don't want to wait to long to upgrade and end up hindering progress.