For your standard single jointed loose ring snaffle, the general rule that I was taught, was that the snaffle can be up to 1/2 inch wider then the total actual width measurement of the horses mouth.
Can be, because some horses will do just fine with a bit that’s the same size as their mouth, but choosing the larger size is a typical choice.
That quarter inch rule if I’m remembering correctly, is to bridle the horse in the bit, with noseband loose, grab hold of the bit rings with your fingers close to the bit cannons, and lightly pull the bit rings apart so that the bit is in it’s neutral position in the mouth. Then the rule says you should be able to fit your finger tips sideways between the bit ring and the lips on each side of the mouth. The sideways finger tip spaces should add up to about a quarter inch on each side.
If you have large fingers, you’ll have to eye the fit or measure it some other way.