Saddle measurements are referred to in two ways, gullet width and bar spread. The gullet is measured in the front of the saddle from one side to the other taken under the swell and is used to determine how the saddle fits over the withers. The bars run on either side of the your horse's back from the front of the saddle to the rear and refer to the angle that they lay along your horse's spine.
Typical gullet widths for a full size horse run form about 6 1/2" to about 7 1/2" (there are some saddles that run smaller and larger). Bars are classified as; semi QH, QH, full QH, Arab, and gaited. Since there is no standardization in the tree making industry, QH bars in one saddle may be semi QH in another.
The best way to measure a horse if you've never done it before is to take the horse to a saddle shop and let them do it. Ask a LOT of questions.
If that is not possible, then go to a hardware store and buy two 2 1/2' pieces of either 12 or 14 gage house wire (the kind they use when they wire your home). Form the first one over your horse's withers and the second one over the middle of his back. Carefully remove the wire and trace them on some heavy cardboard and cutout the shapes you traced. Take the cardboard cutouts to the saddle shop and have them fit a saddle to them.
A poorly fitting saddle can cause a good deal of harm to your horse and is responsible for many behavioral problems that riders have.
Good luck with your saddle hunt!
Btw, judging only by the pics of your horse I agree with kickshaw and would start with a 6 1/2 or 6 3/4 gullet and semi qh bars.