The right bit for your horse is extremely important. The fit is paramount and as any horseman will tell you, bits are for riders not horses.
In general the simpler the bit the better. For fit start off with a medium weight bit of medium depth. The width should extend past the lips of the horse by approximately half an inch on one side ( a pinky finger wide).
Please note for hygiene safety bits are non returnable.
Medium weight loose ring snaffle in a min size at 3.5 inch. Stainless steel....... (click on image for larger picture)