I don't know the actual dimensions, but the battery box will hold two group 31 batteries which are 13L x 6.8W x 9.5"H +/-. The height limits other configurations, such as golf cart batteries.
The settee in front of the galley would work for a third battery. I have the compressor for my fridge there. It is a better location than the shelf in the sail locker where I have my starting battery.