[Japo]

There are two problems with aspect ratios: using one where pixels are square (because its AR matches the physical one of your screen), and the difference between this physical AR and the one of the DOS game, which is fixed. Aspect ratio is the ratio between the horizontal and vertical dimensions of a picture.

Edit the .conf file and replace "fullresolution=original" with another resolution that matches the physical aspect ratio of your screen. To see what choices you have, right-click on the Windows desktop and access the dialogue where you can change the resolution. To know the aspect ratio of each choice, just divide both dimensions with a calculator. To know the physical aspect ratio of your screen, get some tape measure and divide both dimensions in centimeters or inches or whatever.

Still, you may get the same "problem" that the images from DOS games don't cover the whole screen, because each game has its own aspect ratio. 320x200 and 640x400 have 1.6 (16:10) and 640x480 1.33 (4:3).

You can get the the image from DOSBox to fill the whole screen by enabling "aspect=true" in the .conf file. But there is hardly any point in using that option: the anti-aliasing will blur the image, and there's no advantage in enlarging an image across one of the two dimensions by stretching the pixels.