Full cheek bits have long cheek pieces to prevent the bit being pulled through the horses mouth - The Cottage Craft Full Cheek Eggbutt Snaffle differs only in its action from a Fulmer in that the D ring gives a more direct action and eliminates some of the play of the mouth - Especially useful for horses that are unsure of the bit as it doesn't move around much in the mouth.
A Guide To Measuring For A Bit
Measure an old bit you know already fits. Take the mouthpiece measurement from the inside of each cheek piece or ring.
Alternatively use a piece of string about 12” (30cm) long. Tie a knot about 2 inches (5cm) in. Put it through your horse’s mouth, with the knot against the lips on one side. Mark the opposite side with a marker or little piece of tape then take the measurement from the knot to the marking.
A bit should fit your horse’s mouth so that the mouthpeice extends approximately 1/4 inch (0.6 cm) beyond the horse’s lips on either side. It should not be so tight that the horse might be pinched by the rings/cheek pieces and likewise there should not be a great length of bit showing from the sides of the horse’s mouth allowing the bit to slide from side to side. A tiny bit loose is better than too tight. The different parts of the mouthpiece on bits are made to sit on specific areas of the mouth.