[magick-users] resize an image in centimeters?

[pere roca ristol]

  hi everybody,
  I simply need to reduce the length of a very big image; I'm requesting to
a Geographic map service a 1440x720 pixel image so that, when user reduces
the image to a 7,5x3,75 cm (that's more or less 3x1,5 inches) in GIMP or
OpenOffice... it gets about 480 dpi images (for printing).
 I want avoid the user this final step (resizing for including to a
document) and directly serve a 7x3,5 cm image.

  I see that resizing is only useful with pixels... how can I get it? (I
need to request the image with these size in pixels).
  thanks!

  Pere