[Skids]

I would say file size but in some ways that all relates to resolution (higher pixel size as you state) the higher the resolution the larger the file size generally.

Also file type makes a different e.g. JPGs are smaller file sizes then RAW files at the same resolution.

So if PPs claims are true the limitation would be what your computer can cope with which in the main would be dependant on the amount of memory in your computer.

Other factors as well but this is the main one