Quicktime player doesn't work so well with every .swf file I've downloaded. If you get one that doesn't work, try this:
Open a window in your favorite browser. delete whatever is in your address bar. In the finder, click on the .swf file and drag it to the address bar of your browser. This should put the file path of whatever file in where the URLs go. Hit return if necessary.
In Safari, at least, the flash file resizes to fill the window. It isn't as good as the fullscreen you can get with Quicktime Pro but I think the plugin is faster and at least more compatible than Quicktime Player.