Hi! Alt+Enter should take you in and out of full screen :)
Viewing post in Running full screen?
Hmm...sorry that isn't working for you, that is the standard hotkey for game applications on windows (Alt+Enter should work for most games, including Abaddon). You may have your system set to a different key combo for that hotkey? F11 and Command+Ctrl+F are the other common hotkeys but those don't work for me for Abaddon.