Ever wondered how saddles are measured? A saddle must fit both the horse and rider. A small child may not be comfortable in a very long saddle, and an adult may not fit onto a child's shorter saddle. A pony can have a long back, and therefore need a larger saddle, and large horses won't be comfortable in a shorter style.
The length of the saddle is measured from the nail head to the cantle as shown in the picture below.
Saddles come in a range of sizes from 16 1/2 to 21 inches (in increments of 1/2 inch) or in metric sizes from 42 to 53 centimetres, and are sometimes even smaller for ponies. It is not a quick process to find a saddle that fits your horse properly, but the hunt is worth it as a saddle that does not fit will bruise the horse's back and damage the muscles - sometimes irretriveably. Synthetic or second hand saddles will be cheaper than new leather ones. Second hand saddles have the advantage that they are already worn in, and the straps supple. However, it is sometimes best not to set your heart on a particular saddle, as you may find it will not fit your horse, especially if he is not the 'normal' shape.
But take heart - there is a saddle out there somewhere that will suit both your horse and the style of riding you have to do. You may find it may stretch your purse a little further though! But to get the perfect fit for you both, spending the extra on this item of tack is definitely worth it, and will pay dividends in the end. Paying out for vet's bills and back specialists, and having your horse out of work with a bad back will be far worse in the end!