You put the rope or hose in the horse's mouth in the same place where you would put the bit, and then you mark where the horse's lips are. Measure the distance between your marks, add 1/4 inch for fixed or protected rings (full cheek, fulmer, d-ring, eggbutt, goldwing[I think that was the name, it has loose rings that are guarded by metal flares off the mouthpiece so it doesn't pinch] etc) and 1/2 inch for loose rings. I think those are the amounts you add? And then it depends on if the bit you want to use has no joints, one joint, or more than one joint. A bit that breaks will need to be bigger because it will sit curved in the horse's mouth, but with two or more joints it won't use up as much size in the curve so you don't have to add as much.
My boy is in a 5 inch nonjointed kimblewick, and a 5.5 inch French link eggbutt... he is 15.1hh but he has Arab in him so he has a kind of smallish muzzle.