I thought I'd throw in the link to some PerlMagick documentation, since the one listed in CPAN seems to be outdated. You'll be able to do everything using the Perl mods, but thru the command line, you can also use the `convert` utility to change the dimensions of the image and the `identify` utility to find out what they are. It's a really nifty suite of programs!
Good Luck,
  -Adam