You can edit and then adjust the screen resolution for DosBox window in:
Please see Line 27
windowresolution=value - simple change the value as suggested from
320x240 to whatever you want, then save the .conf-file. (Please make a backup copy of the original .conf, before changing anything, just in case!!)
See list of possible value options available:
fullscreen=true for full screen
fullscreen=false for DOSBox in a window
- When using a window, change
windowresolution=1280x1024 (or use 1024x768 for a smaller window)
output=openglnb (uses graphics hardware and maintains crisp image with scaling).
- You might try
output=ddraw, which in some cases has a lower CPU load.
Scroll down to the [render] section
aspect=true (maintains 4:3 aspect ratio, otherwise keep on false if you like the aspect ratio of your screen, mostly 16:9 or 16:10 nowadays)
- Save the configuration file with Ctrl+s (or File - Save)
For more detail please refer to: dosboxWiki | dosboxScreen-resolution | Manual Instructions