Not intentionally trying to be an arse, but maybe take yours out and see what you have? Owners manual? Service manual? Any online retailer that sells batteries and has a cross reference chart? Yuasa website?
There is at least one, if not two posts here on choosing a smaller battery and I believe that they discuss the original larger battery as well. I have replied to at least one of them - search under posts by me or by 'battery' and maybe it will come up (it should).