Oh boy, this is an extremely complicated subject, saddle fit for both horse and rider.
Do you have a tack store nearby that carries English jumping saddles? If you do you are fortunate. Then you MAY be able to arrange getting yourself and your horse down there for a saddle fitting party. IF you are lucky the tack store may have a used jumping saddle that is in good condition, safe to ride in, that fits you, and that fits your horse. Don't count on it though. If the saddler does not have a saddle they may be able to tell you what brand and size can work best for horse and rider (this is a professional level service, be kind, respectful, and do not buy something if it does not feel right to YOU or your horse.)
Just any jumping saddle won't do. The shape of your horse's back, the prominence and width of the horse's spine/withers, how the ribs come out of the horse's spine, the shape, slope & length of the horse's shoulder, the length of the horse's back, and where your horse's "girth groove" is can determine which saddle to try on the horse. There should be ABSOLUTELY NO WEIGHT on the horse's withers or spine. The angle of the panels should be the same angle as the part of the horse's body in rests on, this is CRITICALLY important in the shoulder area. The pommel HAS TO completely clear the horse's withers enough so the rider can get two fingers between the pommel & the top of the horse's withers WHEN SEATED IN THE SADDLE.
Not only that, but as the horse grows and gets stronger through correct training then the saddle you so carefully selected might not fit anymore. Creative use of shims on shimmable pads can help until you can get the panels (the part that touches the back of the horse) re-stuffed to fit your horse's new back.
As for the rider, seated on the horse, your seat should be in the center of the saddle from front to back, you should be able to stand up in the stirrups easily (no heaving yourself forward), make sure that your thighs will still fit in the knee rolls when you shorten your stirrup leather 3 holes, and be able to put the width of your hand between the very front seam of the pommel and your pubic bone, and you want to be able to put the width of your hand between the back end of your seat and the rim of the cantle.
CHEAP LEATHER SADDLES USUALLY ARE NOT SAFE!!!!! Either they are made in Asia with inferior leather, trees, and fittings or they are decently made saddles with major problems (possibly broken trees, or the girth billets need replacing, the stitching is coming undone, leather cracks, loose stirrup bars, etc..)
Synthetic jumping saddles are another option. They are considerably cheaper than a decent leather saddle, and several are made with interchangeable pommel plates (gullets). You still have to make sure that the rest of the saddle fits your horse and that it fits you. While some brands are made in Asia they tend to be much superior to the Asian leather jumping saddles.
If you want a leather jumping saddle but do want to spend the money on an European leather jumping saddle, some Argentine saddlers put out acceptable saddles. The Smith Worthington Saddlery seems to have good ones, and I have heard that they will work hard with you to get the saddle to fit your horse and you as long as you own it.
Do you already know how to jump? If you don't or it has been a while since you jumped I strongly urge you to find a good hunt seat riding teacher and take enough lessons so you have a secure seat over fences. Training a horse to jump when you do not have a secure seat can be quite exciting and upsetting for both horse and rider. I speak through experience here.
The size, in inches, depends on the rider.You measure from the point of your hip to the centre of your knee along the outside of your leg. For me this distance is 18 inches but my saddle is 18.5 inches.
The width of the saddle tree is dependent on the horse. My horse is wide so my saddle is 8X wide. The website below has lots of information on saddles and sizes.