Don't guess, if you get it wrong you've spent needlessly. Put a tape on him & measure from centre of the chest around to a point level with the tail & that's your size.
As different makes of rugs have different fittings what I tend to do is ask the store if I can buy two and bring back the one that does not fit for a full refuund.
Measure the pony as per Toby-Zaphod's reply, then take tape measure to the tack shop and measure the rug too (as they all come up differently just like human clothes)
I've got a fat 12hh pony and he's 5ft in his rugs or 4'9" in a masta as they come up a bit bigger, so I would say yr pony would be about 5', good luck always the pony rugs hang down their chest wotever size you get, always putting darts in front of Toby's rugs.