On the topic of 220's and piston issues - stock pistons were known for that. The 200's dont have that issue. Keep in mind - a 220 is known for great outstanding bottom end power - perfect for woods - but very little on top. A 200 has very decent bottom end, and does better on top.
Size wise - my son at age 12 , 130 ish lbs and 5'7" went to a KDX200 in 2005. His prior bike was a TTR125. We backed off preload, raised forks in the clamps a tad - he was fine until his confidence increased and we set it up normally. Now, at age 15, 175 lbs, size 13 foot - the kid is a tick under 6' - we have gone to bar risers and a tall seat and the KDX is working perfectly for him!
Engine longevity - We bought his KDX used, with at least 1 full season of use on the last top end. (or so the prior owner said - subsequently I think that it had much more use than that on it). We ride quite a bit each year as we have trails on our own property, plus literally at the end of our driveway is a ORV trail leading into over 140 miles of trail accessable from here (we rode 80 miles yesterday in fact). Anyhow - 3 full season of use (estimated at around 3000 miles perhaps more, plus what the prior owner had on it?), we finally did a top end. The bike is running like a champ (not that it felt bad before though).
Powerwise - a buddy of mine, who weighs in at oh 270lbs - he might have dropped some recently - has a KTM 300 and a 96 KDX200. At times, he has a hard time choosing which bike he prefers! He can whip many woods riders I know when he is on the KDX. Look for a DRN user 2big4aKDX on here - that is him.
Back on that 220 piston topic - do some searches here - i dont think it is EVERY 220 has that issue - but many do replace the piston as a preventive measure. But many I am sure, have never worried about it, never new there was any issue - and have no issue.