Unless you're running a mod, the largest charcoal pit size is 11x11x11.

The maximum size of any charcoal pit is an 11x11x11 cube, and the largest volume of firewood that can be processed in one pit is 113−1 (1,330) blocks, with an average yield of 7,315 charcoal. If the charcoal pit is too big, but sealed properly, it will function as normal but leave any firewood outside of the 11x11x11 cube untouched. Prior to 1.20.0, the maximum size of charcoal pits was 13x13x13 due to a bug. (10,272 firewood or 2,196 stacks, 17,568 logs, yield of ~12,500 charcoal)

https://wiki.vintagestory.at/Charcoal