Hi . my boy is 15hh and i use one giant step (1m-my legs arn't very long) for trotting and three giant steps for canter . this was arrived at by trial and error and watching him go over them whilst my son rode him . you could start with these measurements and get afriend to see if his footfalls are in the middle!
if you think of placing poles on the floor the same distance apart as you wold when making bounces for jumping you will get the peract stride, 3 yards apart and 1 yard for trotting.