I believe the size of the display matters to get the best anaglyph experience.
Results on my 20" LCD we’re good, but the distance of the colour offset seemed too great so the effect was slightly uncomfortable and elements in the middle of the screen often appeared double. I read it should be possible to tune the separation distance (using alt key plus mouse movement) but I couldn’t get that to work in RetroArch. Maybe only in the standalone Mednafen VB?
Anyway, the 7" Switch display is perfect, very comfortable and perfect 3D.