‘Height’ in cm = height * screen height
e.g.: 0.5 * 29.3 = 14.65 cm
What I did to check this:
I drew a line with the polygon component, x-size was 0.5.
Based on the formula above the length of the line should be 14.65 cm.
I compared the calculated value with the measured value (I measured the lenght of the line with a ruler on the screen)
How did I get the screen height? I used the program of Morys-Carter, W. L. (2021): Pavlovia
This post was also helpful: From Height to Pixel, mouse coordinates