Its not necessarily the height, but more along the lines of the build of the horse and the movement Is key...a tall horse can do WP. It all depends on training and natural movement. Posted via Mobile Device
As the other posters stated, the particular horses' movement is what determines if it's suitable for WP or HUS, but height is in vogue right now. 16+hh WP horses are becoming the norm (IMO,) but they are being bred to maintain that slow, flowing, straight legged jog & lope. Unfortunately (IMO,) a nicely moving 15hh HUS horse might get lost in the ring of all those currently-trendy 17+hh ones.