Regarding your actual question, I'd suggest fitting as much muffling capacity as you can given space constraints and without exceeding the back pressure specifications for the engine. This later point deserves careful attention. I just went through an exercise exploring the addition of a resonator or second muffler on my boat, but found I would exceed the back-pressure specs if I did, so abandoned the project.
There are various calculator (Harco is the muffler manufacturer in Oregon, USA who I was working with) that will accurately predict the back-pressure from your exhaust piping, mufflers, etc. Personally I like a quiet environment, so it's worth it to me to max out the silencing capacity. And it turns out the builder of my boat had already done that. I didn't pursue it because it would be too big a project, but it's likely that by increasing the diameter of the exhaust run and thereby decreasing its back pressure contribution, you can create more "room" for muffler back pressure. If keeping the boat quiet is important to you, then I'd suggesting spending the time to design the exhaust carefully.
Good luck with the project.
