http://www.ca.dsm.org/forums/ubbthreads.php?ubb=showflat&Number=337695#Post337695I used a 10". I'm really kicking myself now for not actually measuring the volume of water the thing held when I was testing for "leaks" to see if there was enough volume for a 12" or for trying out a ported 10" (would be hell trying to get the port the right dimensions for tuning).
Some time this winter I will take some exterior measurements of the finished product and come up with a close approximation of the volume to see. You could always increase it by coming farther out.
Looking at the pictures again, it looks like a box could actually be made with just the right side trim piece out of the car.
