If it’s a case of quickly putting a fender out to keep the boat from bouncing on a pontoon then you need something reasonably modest. We use 2 that are approx 19″ overall x 5″ dia. A lanyard on each end, one of which has a clip to go on the shrouds. That means they can be very quickly deployed.
Unfortunately when it comes to boat rolling you want something fairly long e.g. 3′ and a diametre ideally less than 1′. For boat rolling the longer the better, but keep the diametre small. In the past people have used drain pipe. It is possible to roll the boat on smaller fenders it’s just a lot more difficult as the boat needs to be kept dead straight otherwise it will fall off the fender.
As you can imagine stowage can be a problem. When we had a mark1A we stowed 2 such big fenders under the forward side benches.
It’s difficult to find fenders long enough that are not too fat. If you are going to do boat rolling then you have to start by getting the boat onto the roller – so a fat roller makes this more difficult.