You can edit and then adjust the screen resolution for DosBox window in:
/home/username/.dosbox/dosbox-0.7x.conf
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:
- Use
fullscreen=true for full screen
- Use
fullscreen=false for DOSBox in a window
- Change
fullresolution=original to fullresolution=desktop
- When using a window, change
windowresolution=original to windowresolution=1280x1024 (or use 1024x768 for a smaller window)
- Change
output=surface to 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
- Change
aspect=false to 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)
(Re)start DOSBox.

For more detail please refer to: dosboxWiki | dosboxScreen-resolution | Manual Instructions