xLaurenOscarx has it right, but I may be able to add a little.
A 13 inch English saddle is TEENY, but English saddles are measured differently than westerns. Measure from one of the buttons on either side of the pommel to the center of the cantle, and the number of inches is the seat size. As a GENERAL rule, your English size will be about 2 inches larger than your western (Ex. I'm comfy in a 14 inch western, but a 16-16.5 inch English). Also, it's more thigh length than rear-end size that counts in English saddles. I'm not sure about the "rule" for that one (maybe just thigh length in inches = saddle seat in inches? At least for me, that correlates. Someone else probably remembers...), but the thigh length is measured from the hip-bone to the center of your kneecap.
Beware cheap saddles! Is there a brand on the underside of the flap, or around the front dee-ring area? Look for a name that you recognize. Non-brand saddles can have really poor quality trees or other construction problems that can
cause harm or discomfort. A lot aren't really well set up to position the rider correctly, either. It's cheaper in the long run to buy a good quality, more expensive saddle, used. Or, you can go with a Wintec if you don't mind synthetics. I'm researching them right now, and have had almost totally positive feedback from lots of review sites, plus posts on this forum.
A pro saddle fitter would be ideal
, to ensure the best possible fit for your horse, but if you're like me, fitters with any more competency than my horse owning friends and neighbors are few and far between. Another pro for the Wintec, IMHO. With a little knowledge about correct fit, you can adjust the Wintec yourself, to a degree.
Is it worth it?? Without seeing it, and assuming it's one of the cheap, poorly made saddles that circulate around, I would say no. A brand name or pictures would be helpful.
EDIT: Sorry, eventerdrew, we posted @ the same time... ;)