It is vital that your horse's tack fits him correctly. Ill-fitting tack can lead to apparent bad behaviour (although it is caused by the horse trying to tell you something is wrong) and it can also lead to health problems, especially back strain.
It is very difficult to advise you on a saddle without seeing your horse and I would strongly advise you to contact a reputable saddler and try out a range of saddles.
As a general rule, you should be able to see through the saddle from back to front once it is on the horse's back. If it fits flush onto the back it is too large and will not protect your horse's back. It will also tend to roll. If it sits up at the back then it is too long and resting on the loins - the worst kind of problem.
The saddle must fit the rider, too, and it may take some time to find the perfect match. Take your time - it's an expensive item and will last you many years if you buy carefully.