When I play the game in single player everything is fine, but when I try to do a multiplayer game whenever I hit Join Game its goes from Fullscreen to a small windowed screen in the top left corner. I've posted an image to show what I'm talking about. Any ideas how to resolve this?

I don't know why it would do that, but you can just select the window and press ALT+ENTER to get it back to full screen.

That is strange. I just tried it myself and found that my join game is grayed out, it cannot be selected at all.
Weird.

Your join game will be greyed out until somebody has created a game that you can join, and your computer (and the game) has acknowledged the connection. If you want to start a game yourself, you must use 'Create Game'. If I am stating the obvious, it is because you never clarified your situation.

Thanks, that tells you how often I've tried it with this game.