First make sure that you're not playing an online game. Our online games are the games you play in a small screen with advertisements. They can't be played full-screen.
If you're sure you have a game that can be played in full screen, you can enlarge or reduce the screen size by going to the in-game OPTIONS menu and selecting LARGE SCREEN or FULL SCREEN. Some titles also allow you to play full screen by pressing Alt+Enter during the game.