To measure the width of the mouth you can use a piece of twine or a wooden dowel about 12 inches long. Put the dowel in the horse’s mouth where the bit would be placed. Mark the dowel or string on each side of the horse’s mouth to get the correct width for the bit. Always round up. If the width is 4 and 7/8th, then get a 5 inch bit, not a 4 and ¾ inch. This mare's mouth measures 5 and three quarters inches - a 6 inch bit would be just about right.
For more information about chosing the correct bit for your horse (including how to determine tongue size and the height of the palate) take the online course Bits, Saddle Fitting and Hoof Balance