I would say it depends on your dressage saddle - my pony takes a 42"-44" gp girth and has a 26" dressage girth - a lot of the dressage girths don't come any shorter than that. So that may be a start!!
they normally say take 20" off your normal girth but saying that make sure you have a long enough dressage girth - if the buckles touch the elbow it can take the horse down. they say needs to be 1" below saddle flap