Ponies vary, saddles vary, I can only recommend you have a fitter out and make sure it sits correctly off the shoulder blade and not beyond the back rib.
Our 11 hand welsh A has a 14.5". It's a little Eclipse synthetic one which is really light so great for wee kids helping to tack up. We started off with a cub saddle with moveable Velcro panels but they were rock solid and I got rid of it pretty sharp.
Our 11hh could only take a 14", and we had an 11.2 who could take a 15" and one 12.2 who could only take a 14.5" and another 12.2 could take a 15". Honestly, best to get a fitter out and get properly checked.
And the underside of a selection of 15" saddles could probably vary by 1-1.5", the only measurement that is relevant for what is the right length for the pony.
I had this dilemma recently. He will get a nice fitted saddle when he is broken, but until he lets a rider on his back to be fitted correctly its a bit of a guessing game! We ended up with a 14.5 MW for 12.2 short backed pony. It looks fine length wise, but my cob saddle pad looked like a quarter sheet! I am hoping when we get one fitted correctly he might be able to have a 15.5, as would like an adult to be able to sit on him every once in a while.
Anything between 14 and 17 inches! If you don't have some to try to gauge then a saddler will be your best bet. Cub saddles only come in 10, 12 and 14 I think.