If you're in to recycling, go to the junkyard and buy the freshest battery they have that you can fit in the car. There's a date code on most batteries, and C8 would indicate it was made in March 2008. If it's more than, say, two years old, I'd pass on it.
Aside from that, a smaller battery will weigh less, but it will give a shorter service life due to being cycled to a greater depth of discharge. Size 51 (Honda Civic) batteries in non-Hondas are fun.
If you've got a hybrid and you need a vestigal 12V battery, why not get one of these:
Braille ''No Weight'' Batteries - JEGS