Essentially Group Size is refers to the physical size of battery that can fit in the area under the hood/in the trunk.
That’s in your owners manual. Personally, I prefer the most CCA I can get in my vehicle’s Group Size. For your car, Rock Auto is showing Group 78 with a range of 690-775. I would go with a 775. They also had an 825 CCA, though that was significantly more expensive (236 vs 158 for the cheapest 775). I would probably stick with 775.