Rein length is down to personal chioce , there are however two things to take into consideration .
1. The reins should be long enough for the rider to be able to ' slip' the reins in the event of a horse tripping , going down a steep bank , jumping downwards etc - the rider should be able to keep a hold of the reins at all times without being yanked forwards.
2. The reins should not be too long - if the reins are too long there is a danger of a riders foot getting caught in the loop formed past the hands .
A rider who is 6'2 and has long legs will be able to have longer reins than a person of 5'2 , even if they are riding the same horse .