Sure some monitors can't do it all. Mine can't keep the aspect ratio. But that doesn't matter if the graphics card can do it. More than a decade ago I had a laptop with integrated AMD graphics (ATI), which couldn't keep aspect ratio. But widescreen laptops were relatively new back then.

So what I'm saying is that for some monitors, the 640x400 doesn't work at all - it's "out of range". Plus 640x400 is typically the wrong aspect ratio - most 320x200 DOS games are intended to be viewed as 4:3, not 16:10, so 640x480 is the proper resolution for dosbox settings to do.

I suggest to upscale all DOS games with integer scaling, so i suggest to upscale 320x200 and all other resolutions to a multiple of it (for all 1920x1080 displays i recommend to upscale 320x200 resolution to 1600x1000, upscale 320x240 to 1280x800 and 640x480 to 1280x960; for all 1920x1200 displays try to scale 320x200 to 1920x1200 but remember to use GPU scaling and to select "centered" in your video driver control panel)

in .conf file use OpenGL as output (NOT OpenGLnb)

remember to use scaler normal3x or tv3x (normal2x or tv2x for 640x480 games if your native resolution is a 1920x1080)

DONT'T use scaler=none

This will wind up effectively stretching the image horizontally for 320x200 games which are meant to be viewed in 4:3 aspect ratio (since that's what almost all CRT monitors were in DOS era.)

Most PC screens when DOs was the usual operating system were 4:3 ratio but the number of pixels and the shapes differed hence the different resolutions encountered in old games.

Most PC graphics cards from 1987 onwards were 640x480 with the introduction of the VGA (and MCGA) standards, the 8514/A allowed 1024x768 but was VERY expensive in 1987.

Before that MDA, CGA and EGA were the main standards (plus Hercules). CGA and EGA were mostly 320x200 if the video card had 64k RAM. MDA only had 4K RAM and was text only - The Hercules could display monochrome graphics.

For the games to have the correct aspect ratio as designed any scaling has to take into account not just the number of pixels but also their SHAPE in the original standards.

Then you have the issue of the original screen size - probably 12". Again 14" screens were very expensive.

Screen and computer video settings also modify what is displayed, as has already been said.

The actual settings for dosbox are therefore down to personal taste. I like to keep the original aspect ratio, but usually this means the image is magnified large enough to fill the screen vertically with black bars at the side on a modern 16:9 screen. What was intended to be a circle must be a circle not stretched to be an oval in my opinion.

Playing in a window the size of the original screens gives the true look of the game better however.

For whatever reason, this fixed my issues with TES Daggerfall not displaying. Thank you very much, regardless!

Integer scaling looks exactly the same. Unless you have a CRT or VERY low dpi screen, playing a game at 320x200 without scaling isn't a very pleasant experience (to put it mildly).

From my past experience, there is no universal dosbox configuration that works the best on every machine. There may be some that generally work quite well for most PC users... but then in some cases you might be better off to change to some other renderer etc.

Especially in cases like this where someone is trying to play on a tablet.