I went and remeasured it today, it was 1 cu ft. which is enough for most 10" and some 12" subs. Its not built to be earth shattering, but to complement a not too crazy sound system. I think it would be nice for someone who just wants a little more than what the car came with without losing any significant part of the trunk.
edit:
Just look at the specs for the 10" and 12" Type R, a very capable and somewhat inexpensive sub (the box enclosure specs are on page 4)
http://vault.alpine-usa.com/products..._SWR-1042D.PDF