Ooh! I have actually experimented with fire scaling myself. To some extent, a scaled down campfire can work, but at a different burn rate. I once made a tiny hearth and experimented for a while at making tiny fires in it. I found that within the range I was trying, a little scale model of a fire that worked well at full scale worked just as well at mini scale, just on a much faster time scale.

If I had a human scale teepee style fire burning, say 1 to 2 inch diameter sticks at about 12 to 18 inches long, I’d need to toss another stick on it roughly every 1 to 5 minutes to maintain a nice blaze. Scaled down to sticks about 1/8th that size, I needed to feed it a stick every few seconds to maintain it. Much smaller and I wouldn’t have been able to move my tweezers fast enough to keep it fed.

The interaction of different stick sizes also scales that way. If was to add a couple of eight inch wide split wedges on that same human scale fire, and once those caught, I could slow down feeding sticks into it to about 1 every 10 minutes, and still keep the same flame size as long as the larger pieces last. Likewise on the same mini fire as above, I could add a few 1 inch wide split wedges, and slow down to feeding it a little stick every 10 seconds.

The size I was playing with could be considered, say, mouse sized. By the time you scaled down to ant sized, the time scale would be so exaggerated as to be meaningless to compare to human sized fires. This isn’t taking into account any additional non-obvious factors that change how it behaves. So no, ant sized campfires are not possible. Rat sized scale campfires are possible, but only if you tend them constantly and quickly.

Also, I didn’t take any precise measurements, I was just making tiny fires to relax. All numbers above are just rough estimates from memory, so you can’t really calculate how the scaling works from this, only that is sort of does work, within limits.