measuring true pixel count

Hello,
For a problem discussed in an other thread, I have a display smaller than the monitor size, though looking as full pixel count.

my monitor is a full HD 23" 1920x1080. xrander say the print is 1920x1080 when what I see is a complete kde screen (including all borders), with very small letters and a black border all around of approx 2cm wide.

I guess my monitor can't display pixels that small and the real display is not 1920x1080.

is there some rule I could display to measure the pixel number?

thanks
jdd