Girths are different sizes for a reason
. You need to measure your horse to determine what size she'll take.
First, put the saddle on the horse. Next, take some string and measure from the second or third hole on the billet strap on one side, under your horse's belly, and up to the second or third hole on the billet strap on the other side. Cut the string to that length, and measure it.
This will give you a good idea of the size girth you need for your dressage saddle. If you buy another type of saddle such as an AP/GP or jumping, you'll have to measure her again since a dressage girth won't be long enough for those kinds of saddles.