Pea puffers get to an inch long and are fine in a 5 gallonI've seen some pea puffers at the petstores get pretty decent sized, bigger than extra large golf balls, though maybe they weren't pea puffers either? I would recommended something a bit bigger than a 15 gallon if you want to split it, just how I would do it.
Contradictory to my comment, I did find something saying that a near minimum size should be 15 inches in length for a single puffer. Hope that gives you a little idea. Though I have found bookshelf tanks that are 12 or 15 gallon with a well over 20in footprint.
I want it to take up the absolute minimum amount of space in my tank. Since 5 g is the min. Can I do only a third of the tank. 12 inches wide 12 inches tall and 8 inches long?
Remember its also 12 inch deep! so its just your viewing area that suffers.You know most of us think more room is always better. 8” just sounds more like a cell than a home. Don’t you think?