2013 Mazdaspeed3 Cargo Dimensions
 

Okay, I have an upcoming road trip and I cas curious to see how much crap I could put into the back. Scoured the net, and could only find the volume in cu ft. If you're trying to determine what size items will fit where, that doesn't help too much. Now if this info is in the manual, sue me. But below are some rough measurements I took with a crappy tape measure. Maybe they'll help you on the odd chance. This is assuming you remove the deck lid and stash it behind the seats with the back seats down.

Measurements

At the Tailgate:
h -- 27 in.
w -- 39 in. (from deck side to deck side, directly under tailgate seal)
d -- 15 in. (how deep into hatch before hitting the max tailgate h at a perpendicular, measured from lower door seal)

From tailgate depth there's about an inch to two before you hit the rear glass. Probably a good spot to tuck in some clothes or blankets/linens.

At the Rear Doors:
h -- 30 in. (from seat back to ceiling)
w -- 48 in. (from seat edge to seat edge)
l1 -- 60 in. (from seat top to the plastic lip, measured down the center)
l2 -- 57 in. (from seat top to the plastic lip, measured down the side)

Remember that any rigid object higher than 20 inches needs to be pushed in about 10-15 inches for the hatch to close. That means you only really have 45-50 inches of cargo length, give or take a few inches depending on how far back you put your seats.

Taking min measurements, I get a volume of 34.734 cu ft. I take this to mean you can fit that amount effortlessly in the back.

Taking combined measurements at the hatch and door, I get a volume of 42.07 cu ft. I take this to mean this is the amount you can actually pack with some finagling.

For convenience, volume as listed:.
cargo volume, with seats down: 42.8 cu ft.
I think with the seats up it is 17.1 cu ft.