plot & print

[Alain Content]

This is most likely a very stupid beginner's question, but still :

I use octave to generate geometrical objects, and need accurate control of 
their size in pixels. 
The way I do that is to plot a figure and then print it.
For instance, to generate, say, a 500 pixel square : 
        plot ([-1,-1,1,1,-1], [-1,1,1,-1,-1])  # a square
        print ("-dpng", "-S500,500", "sq.png")  # generate file

However, this method generates margins around the objects, and I can't find how 
to control the size of those margins (or suppress them).
Alternatively, could one tell me how these margins are sized ?

Thanks in advance, 
Alain