There is a lot of information available - not just limited to jeeps. To address the simple question, the size is defined by your tray and hold down.
Here's just one site that gives you measurements (other sites are out there):
https://www.batteryequivalents.com/bci-battery-group-size-chart.htmlA PO snapped off the tabs of my tray on one side, so I could go with a larger case if I wanted. I chose a common size for availability.
The only other physical feature you'd want to look for is post orientation - so that when sitting in your tray, the cables easily reach.
Googling CCA can help with addressing cranking power. Here's one with information I'd never seen before:
https://www.osbornauto.com/faq-battery-sizeThis suggests for a 134 cu in. engine it doesn't need to be too high.
I learned way back when more CCA's was an upgrade sales tactic. Back then, I'd go lower than the best to help out with my college student bank account.
Since case size is a limit, increasing CCA's suggests more (and thinner) plates to where jarring/vibration could become a factor. Some batteries, e.g. Optima, address this.
When you dig, you'll notice there are other factors that will affect cranking: temperature, oil viscosity, cable condition/size, engine condition (tune up), etc.
I can confirm that a 12V battery will turn a 6V starter real fast. I know this will shorten the starter's life and most likely wear the ring gear more too.
My current battery, bought at the local parts store-just down the street, has 650 CCA's which is overkill. It's price was reasonable and readily available.
