For your needs a 60x120 would be ideal. It would also be an asset when selling your property in the future. That is a standard small ring. Anything smaller would be a turn off if I were shopping. Here in the land of perpetual rain, that size is quite small, 72x185 is more normal for jumping and many boarding barns are in the 100x200 range. The OP is correct that the width really determines the cost, and 60 is the tipping point. Bigger us more $$.
