Some inside dimensions of the original Chubby fire pot would be helpful. Our fellow enthusiast d crane maybe of assistance in regards to the sectional fire bricks as his Crane 44 and 88 uses multiple curved fire bricks. As Sunny Boy stated replacing one brick instead of the entire pot is the way to go.
My other thought would be to contact Tomahawk Foundry and inquire about various size cast iron liners to sit inside the original, just my .02
I have quite a bit of experience reducing firepot size as dad did include an option for cast iron "mini pot" liners for the Crane 404. Its good in theory but in reality its simply not very useful, productive or practical. The Chubby by virtue of being a pure & proper "coal stove" design (cylindrical, deep bed, 100% under grate exposure, etc) can be toned down to a very low burn cycle already, any gains that could be made by introducing a mini pot into it would be far outweighed by the cost & hassle of doing it. Swapping out, storing, cleaning, lifting in, lifting out, etc. are all involved with this process and the simple fact is most people just wont deal with that (even if they have a minipot option). The shape and design of the stove allows it to achieve a very low coal burn already (maybe not as low as true cylindrical, deep bed, baseburner which uses recirculating hot gasses to maintain the min. burn temp required to allow a coal bed to burn through its cycle)... but still VERY low by almost all other standards and a mini pot is just not needed 99% of the time (most would simply throw an oak log in it and turn it down low for those few nights you want to take the "edge" off). The chubby can maintain a 150 degree surface temp for days in fall and spring (if you need lower than that, burn some incense in the room
I do agree for a non-proper coal design (large, flat, non-deep, square) stove, its much more difficult to achieve this slow, low burn cycle for days on end during spring/fall.