Hi
I would like to calculate 0.1 “height units” into cm.

I tried to figure it out via the psychopy demo (Demos > Understanding PsychoPy > Spatial Units.
Here, when I convert 0.5 “height units” into cm then I get 15,82 cm.
However, when I go back to measure the specification with my ruler, the value is not correct. The measured value with my ruler on the screen is 14.7 cm.

Does anybody know the formula to convert height units into cm?

I’ve never used height before, so I’m not familiar with it.
Alternatively, maybe you could try using “deg”?
If you measure the distance between your monitor and viewing location, you could use trig to calculate the DVA you need for the stimulus to be your specified size (in cm).
Image source: Visual Angle - Fast, Accurate, Reliable Eye Tracking

‘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