Create a file called ~/.mozilla/mplayerplug-in.conf
which contains the line :
This will call gmplayer instead of mplayer. This offers a little more options for the video size. But it seems that mozilla and mplayer are not 'aware' of each others existance (if you make the Mozilla window smaller, the video does not scale).
Maybe future versions of mplayerplug-in are better in this area.