How high should I be jumping?

I own a 9-year-old quarter horse gelding. he is 15.3 hands high and is already jumping 3 ft but my question is how high should i jump him? he is very fit and loves to jump and i don't want to ruin that by getting too high and having him bang himself up over too high of jumps please help and email me: bronco_babe500@yahoo.com (so we can countinue the conversation