If you mean amount of all pixels on screen, just press D on your keyboard. It will turn on detailed HUD which shows the amount.
maybe someone could make this a lua script? or did someone already do it? i think so
holy shit you are still working on powder toy, mad respect!
I have no real advice but here is a creation with tools and their capacity :
Wrong channel (correct channel is feedback), and here are them right here: 153 by 96 if you count the full simulation (for debuggers, 14688 pixels).