Majority of people like to play flash animations in full screen, whether it’s a game or any other flash based content. Flash Game Maximizer is a useful firefox addon that lets you maximize and play your flash animations / content in full screen with a hit of a button (Hotkey). You can then resize the window (and the Flash content along with it) in whatever size you like.
How does it work? Install the plugin, check out their About box (Tools/Add-ons/Flash Game Maximizer/[right-click]/About) for some basic usage information. Also take a look at the options (Tools/Add-ons/Flash Game Maximizer/Options).