Sorry I was thinking spreads as in oxers. Whoops.
I think pony stride is 1o ft and horse 12, but im no jumper lolz.
What I used to do to measure a horses stride was find a long run, half a designated start and finish point, and measure the distance between. Then, ride from start to finish and count your strides, then divide number of strides by distance to get your average length. From then, its just trial and error to get the perfect distance between the jumps. Every horse will be different. Or you could teach the pony to lengthen or shorten, as that's the only option you'll have in a comp.