How do I launch a game with a set window size?

When I run a game, how can I have it where it would be windowed and the window size will always be the same when it is started?

You can do this by going into Edit > Project Settings > Player

 * Open up the Resolution and Presentation tab. 
 * Set the 'Display Resolution Dialogue' option to disabled. 
 * Set the 'Default is native resolution' option to false. 
 * Set the desired resolution. 
 * Set the 'Fullscreen Mode' option to Windowed. 
 * Set the 'Resizable Window' option to false. 

And you’re done! This should work just fine.