The firebox total size is 24" wide by 24" deep and 28" tall. However it also has a chimney area attached to that back that is 10"x6".
The door boxes extrude out 12 inches, and are 6"x24" (for the lower one) and the top one is 16"x24". This gives room to build the shed around the door, and be able to put in insulation.
All steel in the firebox and chimney is 1/4" steel.
Inside the firebox, you will also need to weld in the v-bottom plates, and plumb the primary and secondary air. The primary air will go in the front between the two doors, and the secondary will need to be plumbed into the back about 2" from the bottom.
Here is a video to see how this works when it is setup.
When welding the boiler (and especially the firebox), I found it best to fit the bieces together like this:
I did all my welding with a cheap harbor freight welder, and was able to get great welds that are water tight welding like this. I also did a bead on the inside as well.