Hey ya-
I have MEPIS, which by default opens any web videos in a blank window (when I click a link, it does this). I love that fact, as opposed to opening an external player, but the window/video is always kinda small. I can see the video fine, but I would like to be able to scale it bigger. Small and full screen would be two fine options, even.

Any one know how to maximize the mplayer plugin video window?

You can right click inside the plugin and choose full screen if that works for you.
You can right click and choose copy URL and play it in your stand alone Mplayer.

You can use curl to get the address that the media file is sitting at then use wget to dload it.
Or use Mplayer to dump the stream to file. Or just play the .asf link in Mplayer.

I don't think that you can do too much sizing of the Mplayer plugin. If I want to watch the vid larger I dload it and use Kplayer or something to see it. You have to dload it to view it anyway.