The saddle shouldn't be too long ( it should end before his last rib) you should be able to fit 3 fingers between the pommel and withers (while your on him) and get some one to walk him round with you on him and someone else to feel down the side of the saddle too see if it's too tight or pinching. Also you should be able to see daylight through the gullet and the saddle shouldn't be tipping backwards or forwards xx.

I'm afraid I wouldn't mess around, your saddle is the most important peice of tack and could make the difference between a happy, comfortable, safe horse and a miserable, painful, dangerous one!! I would never buy a saddle without having a professional fit it.