Are there any settings to get the biggest screen for GBA, DS and Ps1?
My monitor resolution is 3440x1440 if that matters any.
Settings > video > integer scale ON. This will scale the image as large as possible without distorting the pixels, though it will leave some black borders all around.
If that’s unacceptable, there are many shaders in the ‘interpolation’ directory that will antialias pixels to avoid uneven scrolling / “shimmering”