Mini is 22 inches and wears a medium, I think it is a little bit snug for her but the next size up would be to big. 15hh in cob sized bridle wears an XL. Never measured his head but he is quite dainty.
I use Dinky muzzles for both my cobs but neither were on the edge of a size like that so not sure I can be much help.
My 14.3 cob is XL and my 13.2 is L. I don't remember what their measurements were as it was some time ago, but I do remember there was no doubt which size they were.
They are the only muzzle I've found that my larger cob will just about tolerate and that doesn't immediately rub. Any other muzzle he dedicates his entire being to getting the muzzle off, and if he doesn't succeed gets terrible rubs within a few hours. With the Dinky muzzle he makes a fuss about having it put on but settles down to graze once its on, and he doesn't usually get rubs provided he only wears it for around 8 hours a day and I have two so I can makes sure its clean and dry when I put it on. The downside to them is they do eat through them, they only last about a year for me.
ETA - There are photos of both their heads on the recent "show me your gypsy cobs" thread, my 14.3 has a particularly chunky head.
My 32" mini shetland is in S, she has a dainty head.
The L is a reasonable fit on a 13.2 Fell pony who wore a mix of cob and pony sized bridle (but a noseband that was full sized between the front of the cheeks and pony on the buckle section!)