The video mode is indeed 640x400. (I found that out by measuring the dimension of the DOSBox window.) So I tried different refresh rates for that mode: 60, 70, 72, 75, 85, etc. And 70Hz finally gave me a full monitor-size image. Problem solved. Thanks!

When I run DOXBox 0.60 in full screen mode (with the command-line option -fullscreen), the image doesn't fill the whole area of my monitor. Usually when this happens, it is because my monitor's refresh rate isn't set properly. So I tried changing the refresh rates for all the resolutions from …