A cob or a skinny thing?! I just bought a 52ins for a deep-barreled 16.3, all my other girths which tend to fit eventers of 15hh - 16hh were almost a foot too short. Can you not measure one that is the right size?
It very much depends on your saddle! I have three saddles for one horse and they all use different girth sizes. Sorry. Borrow some and try them for size?
I have a 54 on my IDxTB (who is more on the TB side), it goes about half way up the saddle straps on each side, so could get away with 50-52 I suppose. At a guess I would go for around the 48-50 mark but like said, it does depend on your saddle aswell.
She's 15hh, 6yo slightly on the cob side, but an inbetweeny sort of horse. But boy is she fun lol, she's very responsive, lovely lovely jump which is perfect for me, and hunted lots, hunter trials etc... Her flatwork could do with a little more work tho.