Math question - sort of

I went from the 10D to the 5D. When I had my 10D, I learned to like the
1.6x way of fooling myself into thinking my 400mm lens was a 560mm lens.
Now I am thinking of getting myself another birding camera and am trying
to figure out if there is a way to think of resolution as effective
focal length versus the 10D. So, for example, if I were to get a 7D at
18 megapixels how would that compare to 10D resolution wise in terms of
what focal length lens would I have had to put on the 10D to get a 5
inch tall bird at 20 meters (or any distance) to be rendered by the same
number of pixels (one dimension only or my head will hurt too much) on
the 10D that it would be rendered on the 7D using the 400mm lens.

Eric Miller
www.dyesscreek.com