Most DOS games have a resolution of 320x200, some 640x480 and then there are a few ones with more weird/exotic resolutions.
Only the native resolution taken times a whole number, i.e. 2x, 3x, 5x, will yield pixel perfect results.
So on a 320x200 game it would be these resolutions:
320x200 \ 640x400 \ 1280x800 \ 1600x1000 (FullHD) \ 1920x1200 \ 2240x1400 (2k) \ 2560x1600 \ 3200x2000 \ 3840x2400 (4k) \ 4480x2800 (5k)
Dunno how DOSBox-X handles this, but on the regular DOSBox to have these centered on your screen with black bars around, you will have to add the horizontal number of your monitors native resolution, for example:
Your monitor is FullHD 1920x1080, then the ideal max resolution for the game would be 1600x1000, therefore on the fullscreen setting you'd type 1920x1000.
In your case it should be 1440x800, if the game has 320x200 as base native resolution.
All aspect=true does is change the game from a 16:10 (if 320x200) to a 4:3 ratio, but that results in rectangular pixels instead of perfectly square pixels.
There is lots of different opinions wheter or not to play with the correct aspect ratio. I personally say, if you have a 16:9 monitor simply stay with the 16:10 aspect=false ratio for 320x200px games.