Have a look at the K4 site, not sure how big they go but i just stuck a pair of 16cm fins in my 78lt OES Twin and performance is awesome and only $82 for the pair delivered from Boardzone in NZ.
Flying Objects 165's are a decent size, and they do a G 10 version now too, i can run up to a 6m using them, they have a pretty big surface area which is more important than length.. well thats what the ladies tell me
I recently tried 17s on the chopper 90on a half mast to head high day. They worked Well. Board had more of a single fin feeling but was plenty loose still. This might be my choice for bigger days and or 6.0 weather. I reckon having some 17s and 15.5s to switch around is totally worthy
