Plenty of space and not too difficult to replace. There are two
sizes. It came with the smaller size from the factory (probably
to save weight). Batteries have a size and configuration code and
you just have to look it up at the store (the BCI Group number -- I
don't recall the two numbers for batteries for Volvos). The larger
battery gives you a bit more cranking power. Interstate are good
batteries and so are the Kirkland brand sold at Costco (both
built by Johnson Contorls -- they also make DieHards). See

http://www.johnsoncontrols.com/automotiveBattery/default.asp

Delco also gets good recommendations from people here (and I have to
say the one I had in a Chevy truck was very good).