How do I change my screen size?

i installed a new game and after i quit the game my screen didn't shrink back to its original size so i just wanted to know how to fix it.

please answer quickly, no rush but respond if you can helpHow do I change my screen size?
You have to change the resolution. right click your desktop, go into display settings, and there is the resolution. low-high. most are good 1024 by 768, just play with it to get it to how you had it before.