You should be able to fit between the back of your seat and the top of the cantle for about four fingers. Saddle seats have varying dimensions, too. Slender riders would be more relaxed on a narrow seat with a narrow pelvis. With a wider seat, heavier riders could feel more relaxed.
Evaluate which size of the tree your horse needs. With a standard tree, many horses with specified withers do really well. If your horse is large or broad and has a straight back, a wide tree is required. Farm horses need a tree that's pretty heavy or draught.
If a horse has thin wicks, use an additional pad to fill in the gap. Evaluate if you have the right size for the tree by positioning the saddle on the back of the horse. This is a great match if you can put 2 or 3 fingers in between gullet and its wicks. The tree is too thin if the gap is greater and the tree is too broad if you can only fit a finger or so in the void.
The bars of a saddle are bits of a tree lying on either side of its spine against the back of the horse. The rails should be at the perfect angle so that the bar offers enough force and no spaces on the back of the horse. A better fit is crucial because while you are moving, the bars evenly release your weight.
Highly trainable horses would need a tree that is built for a gaited horse, such as Tennessee walkers and fox trotters. In all of these breeds, certain trees have a larger gullet that is normal. In fact, they have a flare in the front bars to provide wide range of motion to the horse's shoulder and much more rock, or circumference, in the bars to match to the horse's back.