How to Change Resolution and Windowed Mode
Right click on the game on your Steam list. Or click on the little “gear” icon (called “Manage”) at the game page on your Steam library.
Select Properties from the context menu.
Click on Set launch options button.
Type in your desired resolution, and if you want full-screen (1) or windowed mode (0). See example below:
-screen-width 1280 -screen-height 720 -screen-fullscreen 0