I have seen that in using
:thumb => "100x100#"
:medium => "480x320>"
These follow the rules of formatting for ImageMagick which can be found here
> lets the image keep the same aspect ratio when it is being scaled so it doesn't get distorted.
You will not find the
# key in the ImageMagick docs though, that is special to paperclip. It allows simple cropping of thumbnails to the specified size. See here for a good explanation, about halfway down the page.