The dimensions of the original WLM scenes are 640x320.
You can have a look at what WLM did to your photo as you imported it at :
C:\Documents and Settings\%USERNAME%\Local Settings\Application Data\Microsoft\Messenger\YOUR-EMAIL\ObjectStore\Scenes
(Copy-paste the latest *.id2 file after the import to your desktop and change it *.png or just open it with any photo editing software.)
If your picture is to big then WLM will cut out a section an convert it to 640x320.
But like djdannyp said just mess around a little