I think in your case, additional length would be a more valuable asset, than diameter... Perhaps a few 12 x 42s would do nicely?

My 12 x 29s are definitely overkill for my boat, for normal use I'd probably be happier with 10" diameter... When lying on a floating dock, or a wall, and hung vertically, they're fine... But, when used in conjunction with a fender board, and lying off a piling not flush with the face of a fixed dock, they can place the boat a LONG way outboard, making for a long step from one to the other... Using 18" diameter fenders, I can easily imagine situations like that, where you might almost need a gangplank of some sort to get on and off the boat...

Extra-long fenders can really be nice to have, and when hung horizontally can often eliminate the need for a fender board, or the use of more than one fender... I found this old one made by Avon in a consignment shop years ago, it's come in very handy on occasion...