controlling aspect ratio

I need to control aspect ratio of the plot as visible on the screen or printed.. for example, a plotted circle or ellipse must come out with the right aspect ratio. (Note: printing is primary, we assume printer pixels are perfectly square, and we don't care if the screen pixels are not quite square.)

(Note to John -- I'm in Ted Drain's group at JPL, and I'm looking at this early.. I know you've been hired to create an ellipse primitive.)

So what would be a way of controlling aspect ratio? Can this be done via pylab, or better done through the OO interface? I know that I can resize the figure canvas, but to be strictly correct I need to know the length of the axes in pixels, not the figure canvas.