It's easier than you think. Just set up a tag with brackets ][, and you can add some H=W= parameters to make it fit nicely. It'll be second nature before you know it. Until there's an easier way...

DO THIS:

  [flash http://www.youtube.com/watch?v=SOME_CHARACTERS]

For height and width action,

  [flash http://www.youtube.com/watch?v=SOME_CHARACTERS W=240 H=160]

May I recommend 240x160, or if it's really important, 320x240.

Center all that shit in WordPress and you're SET.

Don't believe me? Read the sacred text for the plug-in itself.