Re: Holding tank size
It will of course also depend on your head - i.e. an electric head with a macerator that uses either fresh or salt water as part of it's flush can take anywhere from 1.3 quarts to 1 gallon per flush (presumably for 'solid' flush only, but who knows).
Go big, but be mindful of the structural issues and weight. 400 extra pounds forward or aft can seriously affect the waterline and under body profile of the boat.
Lessons learned are opportunities earned.