Hello @adrian_buechli
That’s a difficult topic. See, for instance, Blurry / low resolution online - #26 by nickpoth21. Using height instead of pixels as a size unit seems to alleviate the problem slightly. Various devices differ in screen resolution and consequently in pixel size. When designing webpages, you use the screen resolution to display the variant of your image/graphic that best fits the screen resolution, rather than letting the browser make the necessary adjustments.
See the section on resizing the window Bringing experiment to full screen after some loops
That code should make the window full-size.
psychoJS.window.fullscr = true;
psychoJS.window.adjustScreenSize();
Best wishes Jens