[pygame] How to control the physical size of displayed objects?

[Mathieu Dubois]

Hello,

I'm working on a program (written by someone else) to use PyGame to 
display some images for use in a psychological experiment. I am new to 
PyGame.

For the experiment we need to display circles of a given physical size 
(let's that the diameter must be 30mm). Currently the program seems to 
do the conversion with a hardcoded value.

I wanted to know if it possible to compute this value automatically.

If I understand correctly, I need to know the physical size of the 
screen and the resolution. Knowing the resolution is simple with PyGame 
but how can I access the screen size?

The program should be multi-platform.

Any advice?

Thanks in advance,
Mathieu