At the moment smplayer doesn't seem to have any xinerama support, which makes working with multiple heads a bit uncomfortable.
Personally, I'm using nvidia's twinview with two heads: an LCD (1280x1024) and a TV (1024x768). Mplayer detects this setting and scales the picture down so that it fits on the TV. Smplayer doesn't do this yet.
If not by implementing xinerama support, this could be done by adding an option to use a portion of the current resolution (1024x768 out of 1280x1024 in my case). I for one would really appreciate such a feature.