Need to know what size of girth to get?

How much too big is your current girth? Measure the girth, then subtract how many inches it's too big and that's your new girth number. Otherwise, you could put the saddle on, then measure where the girth would go, from central hole to central hole on the billets. If it's in between sizes, go with the smaller one, since it will fit tighter when tightened up. There is no reliable and direct conversion from western to english girth sizes, since the build of the saddle can make a difference where the girth/cinch hooks onto the saddle (like some hook on higher and some come down lower).