So you may have a dilemma. You want your horse to get some fresh air and exercise but you don’t want him eating grass and getting sick. Your solution is the grazing muzzle and here’s how to size one for your horse according to Christine Barakat of Equus Magazine:
How they work
Looking much like a basket, grazing muzzles are made of mesh and have a small hole on the bottom allowing the horse to drink and eat a much smaller amount of lush grass. They will still be able to breathe normally.
When it’s not right
If you put the muzzle on and you can loosely tip it back and forth, it is much too big and will be ineffective. If you can barely get your finger in-between the muzzle and the horse’s nose, it’s much too small and will cause horrible chafing.
The perfect fit
There are a few things required for the muzzle to have a perfect fit. You should not be able to pull the muzzle forward or backward off the nose but for a comfortable fit there should be at least an inch of clearance from the bottom of the muzzle to the nose. Also, check to see how the halter fits; it should never be too tight around the jaw.
Source: Equus