This will work for all games on dosbox in low resolutions.
mostly what happens is that u see the game in the middle with alot of black borders around.
in order to fix it and force the screen to stretch it u need to do the following:

go to NVIDIA CONTROL PANEL by right clicking anyhwere on desktop.
In nvidia control panel go to "Display".
2. choose Adjust desktop size and position page.
choose Use Nvidia to change size instead of the "keep aspect ration" option which makes the black borders (the 2nd one).

This also works for win 95 games like red alert 1 and C&C!!
no more config tweaking.

U can go to ATI desktop options and change it as well.
Go to Graphics menu - desktops and displays- click on the traingle on the bottom- configure.
and Enable GPU scaling.

more here:
http://support.amd.com/us/kbarticles/Pages/UnableToSetGPUScaling.aspx