I have a 26 ft sailboat. I picked up a good condition spinnaker from a 30 foot boat on the cheap. Will it work?
A triradial spinn could be shortened in the mid panels.., or the top and bottom of the radials.. - I'd think removing one of the central panels would do in this case, at least get you a lot closer to a decent fit.It depends on the construction of the sail. Tri cut sails are much more difficult than crosscut, but either can be done.
There is an easy way to discover the answer to your question: try flying it.
You can fly a much larger spinnaker, either symmetrical or asymmetrical than the racing rules allow. Let your experience determine the size you can fly, not arbitrary racing rules. If you examine older photos of boats flying spinnakers, you will see spinnaker poles much longer than the "J" measurement, luffs much longer than "I" + 10%, and a foot longer than 1.5/1.8 x J. Go for it (as long as you are not racing in violation of the rules or assigned rating without a penalty)!