I have a 14.3 hh horse atm, and he uses about 3 ft for walk, 4ft for normal trot, 4.5 ft for a bit longer steps. We do not want to hold them back, but stretch out and move forwards, so 4 and 4 1/2 ft is the best distance at the moment. I usually test at first and then you see, where they step, how they go, if they hit them too often etc. Some might even be able to trot wider ones, but 4-5 ft is the standard trot pole distance for different sized horses. Ponies might only be able to stretch for 3 ft.