For a stocker A12 a 350CCA lead acid battey should be more than enough, and they are about the same size as in the pic.

Im sure you could run with a 280cca or less, but you would want to make sure it starts easy all the time. One carby flood and 30 seconds on the starter would drain a small battery pretty quick.

The Calcium-calcumim hybrid batterys seem to about the best ive found for compact power, but they are alittle pricy. I got a 3 year warrenty with all the ones ive bought, (350cca and 800cca units), so i reckon they are good value.