Roughly 3 big strides. This means you can do walk trot and canter over the poles. You can adjust them to longer or shorter after a trot/canter over them.
my horse is 17.1 and i do roughly two big strides and a foot.....Makes hi think about whatb he's doing a works a little harder.....then I enlist a little sister to move them as required..!!
Surely it depends on the height of the human as well as the height of the horse? I'm 5'3" and used to do 3 decent strides (not stretching!) between my trotting poles for my 17.2hh but now I use 2 stretchy strides for my 15.1hh Arab. If I was Jodie Kidd on the other hand I reckon I would only need 2 stretch strides for the 17.2hh and maybe one and a bit for the 15.1hh
Giant step then half a normal, for a 15.2 is horse its around 2 of your strides, or like me just one huge one, but for bigger horses I do one huge one and then half a one, but it depends on your horse and you I guess, you'll need to do some adjustments.
Just having visions of everyone doing these strange manoeuvres in their outdoor schools, what with fairy steps...giant steps etc, anyone non horsey watching us would have us marched off with men in white coats and locked up lol