Are we talking engine starting batteries or house batteries? They have different requirements.
Since you said two batteries, I'm guessing it's the starting batteries. If you have the ISL engine, you need two batteries that can deliver at least 950 Cold Cranking Amps (CCA) each. They will probably be the group 31 size to have that much power. If you just ask for a "big battery", it probably won't be anywhere near that much CCA rating.
2004 American Tradition; 2014 Buick LaCRosse
Homebase in the Ocala Nat'l Forest near Ocala, FL