As a general rule, we would recommend for you to base the size of your fly mask on the size of your horse’s bridle. This is certainly the case for the Field Relief range of fly masks, but will also give you a good starting point for other brands.
Regardless of make, your fly mask should fit snugly (two fingers) around your horse’s face with no gaps over the poll or around the nose. Excessive space in these areas will make it easier for the fly mask to shift on your horse’s head, or pull off completely. To help with this, look for a fly mask with stretch seams and plenty of adjustability; the Field Relief range all feature stretch seams for a secure fit, with the Midi (with and without ears) also boasting hook and loop adjustment over the poll as well as under the cheeks for a more tailored fit.