My feeling would be that for 100 square feet, you would not want any system with a major "instant heat" component like a barrel. Just the exposure of the feed tube or firebox door would keep you warm enough while the mass accumulated and radiated heat.
As size is critical here, I would consider a 4" batch box inside a masonry bell, which could be 2' x 3' or a bit less. If you want a bench for sitting/sleeping on (probably wise), the bell part could even be a bit smaller, and connected to a half-barrel bell cavity in the bench. The total internal surface area (excluding floor) of the bell(s) is important for extracting the right amount of heat from the fire for efficiency and good function. The website
batchrocket.eu has a table for this sizing, and while 4" is not included, careful extrapolation suggests about 25 square feet of ISA. This would be an internal space around 2' x 6' x 1' high, which after adding wall thicknesses and accounting for the taller initial bell would give a bench 3' x 6' in addition to a bell over the core 2' x 2' x 2' taller than the bench.
A 4" batch box would be easy to build from firebricks, preferably "splits" that are 4 1/2" x 9" x 1 1/4" thick. I recently assembled one as a test which worked well and required no brick cutting:
https://permies.com/t/59459/EZ-Tools-Brick-Micro-Batch#505805