Personally, just as with the "big horses" (and ponies of all heights), you really have to figure out the stride of your individual horse. Only once you know that will you be able to "match" it to "stepping off" the distances. And once you know that stride length - you teach your horse to shorten and extend that stride.
As to the striding/distances at ASPC/AMHR shows - the handful of shows I've been to all seem to set the jumps at different distances and I've not figured it out. I would have to go back thru the rule book for the jumping classes. I thought the jumps were to be laid out at specific distances from each other...
My own walking/strides have changed thru my life. When I was walking jump distances, I "strode" (is this a word?) a set length (don't remember what it was - but REALLY THINK I matched it to each individual horse and sometimes rode up to 4 different horses in jumping classes). Then when I joined the Army, we marched - again at a set "stride length" (30"). Now @ 50, out of shape and 100+ lbs overweight, I don't come anywhere close to that! And when I actually spend time "extending" my stride - my legs let me know how UN-HAP-PY they are - right around 2 am, LOL.
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The pony that "started it all" for us in 1995 was 45 1/4" tall (11.1 hh). His stride, when in shape and fit, rivaled/equaled that of many 12.2 - 13.2 hh hunter bred welshies & welsh Xs in our area! He sired that stride on all of his foals - commensurate w/ their heights (YES, the 14hh 1/2 arab gelding had a bigger stride than he did) - and his 2 daughters that I know have been used for breeding (purebred shetlands) have also continued to produce that stride. His smallest foal, out of our mini shetland mare, matured at 36" in height - yet could match his stride at times (couldn't hold it as long - especially when not fit). Yes, fitness as well as health make a difference in individual stride length.
Here are a couple of pics of his grandson (silver spot) and his last son (black spot) - with a vast difference in height - but striding the same in these two pasture shots (it's not showing an extended trot, just stepping the same). The silver dapple gelding can not hold that length of stride for as long as his larger "cousin", though... I don't actually have any good "trot" photos of either of these ponies. I hope I'm wrong and that our 2 granddaughters will become interested in riding. Both this silver dapple/white gelding and the black/white colt would make excellent riding/jumping ponies just like their sire/grand sire.
Can't find the other shot showing their striding together... But this one demos the height difference. They are a week apart in age.
We often gave lessons on ponies varying from 9.2 hh to 14 hh at the same time. If cavaletti set up, then the ponies learned w/ time, how to shorten and lengthen their strides. We changed the spacing between the cavaletti to sometimes be set for the smallest pony and sometimes for the largest pony. They all learned how to move thru them w/o falling on their noses.
The stride length at a trot is different than the stride length at a canter/lope. And sometimes the pony w/ the longest trot stride will have the shortest/choppiest stride at the canter, LOL. Cavaletti work at the trot will improve the stride at the canter.
