It's quite confusing to get DOSBox to display games properly on modern hardware, partially because CRT monitors often displayed in non-square pixel modes, and partially because modern LCD displays don't scale well or support all the resolutions that CRTs did.
This post discusses the settings required in dosbox.conf to get DOS games displaying well.
Set this to your screen's native resolution such as fullresolution=1920x1200
The default surface cannot scale, so set this to output=overlay which can scale. If you have a decent graphics card you could also try output=openglnb
Set aspect=true to ensure the game is not stretched
Simple scaler=none will work, but you can experiment with the other scalers to see which performs well and which you like the look of.
The full config options are described at http://www.dosbox.com/wiki/dosbox.conf