Here is my box/setup in the rear of my 73 wagon. Surprisingly the pre-built 10" sub box I bought from Walmart fit perfectly and sung between the 6x9 enclosures. I was expecting it to be either just barely to big or slightly small, but it's not tight, but snug enough to keep it from sliding around! I didn't take any measurements of anything either, just cut the 6x9 enclosures to what I felt comfortable with.
I agree with Pintopower, the 6x9s sound great in the back by themselves, I too found it surprising how good it sounded without the sub, but I like to have some bass for when I'm listening to some newer rock or techno (Trance to be accurate). Without the sub, I found myself anticipating some sort of bass drop when the beat started but it all sounded to empty.
For anything 90s or earlier when it comes to alternative, classic rock, or even country (newer too), the lack of a subwoofer is fine, as the 6x9s fill the cabin with sound nicely and evenly.