The bike is 91. At a glance the bike could pass as a 17 or 18, but again, they didn't make 17 then. My size 18 92 Explosif also looks "small" to me for an 18", but I think that's due to the shorter top tube length as compared to my 18" King Kahuna from 2000. The 90 Cinder Cone also looks big for an 18, but I measure 18.5" c-t. The 88-90 frames differ from the early 90 ones before they were suspension corrected. And again the geometry changed slightly after this point. I would say it must be an 18", but I still think you will have no problem riding it. It's definitely smaller than your other frame.