Canvas size limit and pixel settings

[Jacob22237142n07j]

I'm still pretty new to digital art and fresco so I'm not sure what the pixel settings should be for different sizes. I'm not sure what ppi is or ppm. I'm trying to make a canvas of 0.8cm x 0.8cm, but it turns into 0.846cm everytime I enter it so it makes me think there's a size limit to how small I can make it. Any help is appreciated. 

[DaveJCummings · Accepted solution]

PPI is Pixel per inch, much like DPI, and PPM is Pixel or points per milimeter so the higher it is, the higher resolution it is.  .8cm is a really small image, so its probably limited to keep things too pixelated.  Up the PPI and see if that helps