ScummVM's version of SDL uses the windib driver by default. On my machine this means that it uses a 640x480 resolution in fullscreen mode. This gives the wrong aspect ratio for 320x200 games (Lucasarts adventures etc) and aspect ratio correction results in horrible scaling artefacts 🙁

However, I can force SDL to use the directx driver as follows:

set SDL_VIDEODRIVER=directx

This gives me 640x400 (correct aspect) in fullscreen ScummVM if a 2x scaler is selected, or 320x200 (correct aspect) if no scaling is selected 😀

This will probably only be of interest if you are going for the DOS experience, e.g.

a) Using a CRT monitor (as otherwise your TFT will be doing scaling)
b) Not using any filters (as otherwise you'll want a higher resolution)