Is not only the tank.
You would also need to find space for a sealed locker, vented in a donward direction to the outside, where you would need to set the tank in.Before getting into tank sizes and specifics I think you should analyze and determine your type of sailing and cooking needs. A live aboard on a mooring or a full time cruiser will have different needs that someone who does weekends. As a compromise production boats are usually being equiped with two 10 lbs tanks stored in locker located inside a cockpit lazarette. This is a compromise and not necessarily the best setup for you. I would say use the smallest tank that you can conveniently get away with. Consider refilling logistics and convenience.It is not necessary two have a dual tank system in order to have backup as there is an adapter available that allows you to use 1 lb disposable canister with your propane system. The beauty is that in the locker space for a single tank you can always store a couple of these 1 lb canisters with it. This results in a smaller locker being required than for a double system.Follow all the safety guidances for installation. Thirty years ago it was not unusual to see propane tanks hanging from the stern rail of boats being connected to stoves via copper tubing and with the tank valve serving as the only gas interrupt. Nowdays people want instant gas available at the stoves and boats with slick lines. Propane has become the fuel of choice and it is these new safety guidelines that have kept the incidence of accidents down.