If the numnah is pony size it isn't likely to fit a 17" saddle i wouldn't have thought as that is a horse size saddle (in my opinion!) which would probably need a cob or full size numnah?
If you have already paid for it now, just try it, and send it back if it's no good. I have a pony / cob size numnah on a 16.5" saddle, and that would probably *just* go underneath a 17" saddle - but I guess it depends a little on the shape of your saddle.