How to have real full screen with Civilization in DOSBox?
You probably will need to change the scaler parameter - this controls how the "classic" DOS resolution is upscaled to fill the higher resolution of today's monitors. The various types represent the algorithm used to upscale the image, and the "2x" or "3x" control how much upscaling should be applied.
You may also have to change fullresolution, which does additional scaling to match your monitor's resolution.
Try using
fullresolution=1366x768
output=overlay
scaler=normal2x
Now try to play it fullscreen.