Girth - depends on how fat he is ! Our 15.1 is currently in a 51 inch non stretchy cottage craft one which gets done up high on the girth straps, a smaller one would easily fit him eg. 48.
Bits you measure between the rings but they often have the size written on them. They actually do vary a bit actually as I've just measured our twos bits in view of getting new ones and one is a happy mouth mullen which says 135mm/5 and a quarter inch the other a metal eggbut that says 5 and a half inch but they are pretty much the same size.