I have a 1988 Seaswirl sable. I have a 350 inboard with a cobra outboard. What size kicker should I buy for backup propulsion in case my engine dies and can use for trolling as well?
Yep......as previously stated. And I believe with your hull type you may want to link the kicker to the out drive to permit steering from the helm when trolling.
Anything in the 9.9 or larger range will work fine, if you plan to get a somewhat newer motor try to find a high thrust model, then an 8 or bigger will work well. You don't need anything bigger than 15.