If you are trying to play a game using a lower resolution as your display, try to keep your aspect ratio, so the image is not stretched. For example, if you have a fullHD display (1920x1080), try to set a 16:9 resolution, as:
1600x900
1366x768 (or 1360x768)
1280x720
1024x576
960x540
There are some games that let you select the aspect ratio separately from resolution, so in that case you could use any resolution, but only select the 16:9.