To get a good world view with that is similar to a full screen when using /viewport, try to keep the x divided by y equal to 1.33. Many full screen resolutions correspond to this, and perhaps this ratio can be considered the default for least distortion.
Full screen examples:
640 / 480 = 1.33
800 / 600 = 1.33
1024 / 768 = 1.33
1280 / 960 = 1.33
1600 / 1200 = 1.33