The best person to ask would be your saddle fitter, there are many things to consider when fitting a saddle, the height of the pony is only one of them.
You can check for yourself with a given saddle but you really shouldn't try and guess . Once you have a saddle to try, feel forwards from the soft part of the flank in front of the hips, and you should come to the last rib - track it up and forwards, and then feel where is disappears under the muscle that the saddle sits on - this is the length you have available, the saddle shouldn't extend beyond that point. Bear in mind of course the saddle must be well back behind the shoulder (tree points about 2" behind the scapula minimum) and that even different models of a given brand can vary in how long the panel is compared to the seat size.
I have horses of that size wearing from 16" (rare!) up to 18" (also rare). Most likely would be 16.5-17" but our saddles have fairly short panels (www.nativeponysaddles.com).