What do you mean...? A web browser window...?
If so: Make a new bookmark (preferably in your Bookmarks bar) and call it Full screen (or just [F] as mine is called), then edit the address/location to:
javascript:self.moveTo(0,0);self.resizeTo(screen.availWidth,screen.availHeight);
And, voilà, you have a Full screen button.
Now, don't overuse it. Full screen mode is not good for you... 😉