Dimensions:
Without scaling, the plane is about three times and
a half as wide as your avatar's height.
For Developers:
The screen has a 4:3 Width-to-Height Aspect Ratio so the texture
should take that into account in order to avoid stretching.
In other words, you should use a picture that is (approximately)
4/3 as wide as its height, for example 800x600 or 400x300...
Note: If you don't need transparency, simply set the opacity map
to pure white.