Bit measurement is taken in a straight line between the rings or eggbutt, of the bit.
See size guide below for horse breeds and sizes.
Check your horse's size but these are the most likely fit
Ponies 9 Hands -11hands high = 4inch (10cm)
Horses 11.1 Hands – to 14 Hands high = 4.5 inch (11.5cm)
Horses 14.1 Hands to 16 hands = 5inch (12.5cm)
Thoroughbreds, Stock Horse from 15 Hands - 17 Hands = 5inch (12.5cm)
Warmbloods 16 Hands - 18 Hands = 5.5 inch (13.5cm)
Clydesdales 16.2 Hands - 18 Hands = 6 inch (14.5cm)