The answer really depends on whether or not you are using the command line or a wrapper to do the resizing. If you are using the command line version, then call convert.exe sourcefilename -resize 450x450 outputfilename. If you want to do this through a third party wrapper, you are going to have to read the documentation for that wrapper.
You can find out more about resizing with ImageMagick in
the rather comprehensive documentation[
^].