If this is for the house side of things, usually "the bigger the better". But it's totally a personal preference based on how you use the boat. If you're talking about the engines, this has nothing to do with the boat. Find the minimum specs for whatever engines you have and go from there (power, not necessarily group size) - check the manual for your engines.
Group 27 Deep Cycle Marine
Foot Print is 6.5" x 12"
Height is 9" to top of post
Cranking Amps @ 32 is 810
Cold Cranking Amps @ 0 is 650
Reserve Capacity Rating is 150
Unless something has changed, and this was true of 2016 GWs I saw last weekend, the stock batteries are Penn Deka group 27. More importantly, they should be cranking rather than deep cycle. The plates are designed for different use cases. This has been discussed many times on various message boards.
Some folks add a deep cycle after purchase for powering stereos and such. I did this on my 272 and will likely have it done if I get a new 300. Cranking batteries are designed to supply amerage differently than deep cycle. There are dual purpose batteries but I am pretty sure Yamaha strongly recommends cranking for their engines.
BTW: guess who is the worlds largest purchaser of Yamaha engines?