There was a thread similar to this yesterday which you may find helpful - I would go for about a 26 inch but until you get the saddle it will be difficult to be exact! My 17 hand WB dressage horse take a 28 inch dressage girth and his normal girth is a 56 inch, my 16.2 WB takes a 26 inch.
I didnt have a clue either when I bought my first one, so I tied a length of baler twine from one girth strap under the belly to the other tied a knot in the two ends and took it too my tack shop and said have u got a girth this size! didnt look at me too wierdly!