What is the native res of your LCD? Post the contents of your dosbox.conf

dosbox config:
fullresolution=0x0
output=ddraw #or opengl, openglnb, or overlay
aspect=true

edit - explanation:

The fullresolution line sets dosbox to use your desktop resolution (which most probably is the right one to use with your monitor). The output line enables hardware scaling so dosbox stretches the game (from 320x200 pixels or whatever) to fit the screen (with output=surface, dosbox adds black bars instead of stretching). Aspect line makes sure the game has correct proportions (not too skinny or fat graphics).

Thanks ripa, that solved SW. Now I'm going to test Ascendancy.

That should fix every game except text mode games (aspect correction doesn't apply to text mode so you'll have black bars on top and bottom with a 4:3 monitor).