A lot depends on what you want out of a battery for your particular application. If you are just using it to start the motor then that is one thing but if you are running a lot of accessories as well you may want to consider dual batteries and a switch between them.
A liitle more info will get you a lot better answer.
By the way, welcome to iboats.
My boat has a 70hp and only had Navigation Lights and Bilge pump. It came with a size 24 battery. Make sure you don't get a too small of a battery, because you need the engine to spin fast enough for it to start.
