To determine the tree size before purchasing a saddle, you need a tracing of your horses whithers. Instead of using an old coat hanger, a 24" flexible curve or "Curvex" is more accurate and easier to use. You can usually purchase one in a good office supply store.

Stand your horse square, on level ground. Place the "curve" just behind the shoulder blade and carefully mold it to your horses wither shape. Carefully lift the curve off the horse's back, place
it on a piece of paper or cardboard and trace the inside edge with a pen. If you take this to a tack store, try fitting the cardboard cutout under the pommel of the saddle. Allowing for rider weight, you should have 3 1/2 to 4 fingers above the cardboard under the pommel. Less clearance means the tree is too wide and more clearance means the tree is too narrow.