fullscreen resolution

Thanks for a great tool for online experiments!

I face an issue with resolution in a fullscreen mode. 

Whatever is my resolution set in the options the resolution is set the same
for a given computer, e.g. 1536 x 864 or 1280 x 720 (with is in many cases
less than the system Display settings, e.g. 1920 x 1080).

If the resolution is set more than these automatically established value, like 1920 x 1080
under automatically set fullscreen resolution 1536 x 864, the images which are larger this
1536 x 864 simply truncated.

If the resolution is set smaller, e.g. resolution of 800 x 600, then the screen resolution says unchanged,
but only the central zone of 800 x 600 is visible, like in the printscreen (the grids are 100 x 100 pix).
 


My goal is that the image I show occupies a relatively similar portion of the screen for different computers,
but I can not figure out an optimal strategy for scaling under the restrictions I describe here.

Thanks a lot for any suggestions!

Sincerely yours,
Volodymyr