The numbers behind the lowest sized font for a given printer resolution
1. Choose a pixel font. The one that uses the smallest amount of pixels to define the line height is 5px, plus one pixel for the line spacing it gives 6px. (only for standard english alphabet) There are some other models that need some more pixels per line.
Pixel fonts examples.
2. A point (pt) is 1/72 of an inch.
3. We take the DPI of our printer, in this case, 600 DPI and divide it between 72 to see how many points we can have on 1 pt. 600/72=8.33 We can have then 8 pixels to play with our fonts.
4. This also means that we can have (potentially) a font size lower than 1pt. We can have a font of 1/100 of an inch.
5. On a higher resolution printer, we can have lower sizes, but some other factors come into play, like absorption of the paper, dot gain, etc.
The extra resolution can be used to "solidify" the dot. Instead of printing a smaller font, it can use 4 dots instead of 1 to define the black and the white spaces.
The old answer:
My 2 cents.
For really small fonts on commercial offset print, I would make a test on the press itself using well-controlled 1-bit images using pixel fonts at various resolutions, mainly 2400, 1200 and probably 600 PPI.
Make them one ink only, on a CMYK file, it would be only one of those colors or one additional spot color.
Do not use a big offset printer (like A0) but a high precision small one. (Like A3) because on different sections of the cylinders, you can have different quality on the print. If I need to use a big machine I would use a small paper.
Use a synthetic paper.
I would not use negatives but direct to plate.