Question: how to change screen resolution in league of legends out of game?
Answer: Browse to C:\Program Files (x86)\GarenaLoL\GameData\Apps\LoL\Game\Config.
Open "Game.cfg" in notepad or any other text editor.
Find this line: WindowMode=X.
Change the "X" to 2. IE: WindowMode=2.
Save this, then launch a custom game.