It’s all in the title= I don’t remember if xxx_width and xxx_height are
magic columns that paperclip would automatically fill if ImageMagick can
handle the upload. It seems no to me, but that would be so easy to
add…
You might want to try the paperclip mailing list:
http://groups.google.com/group/paperclip-plugin
I do remember coming across someone who had made a fork that stores &
makes available the dimensions, but for the life of me I can’t find it
now. If you find it, let me know 
I could use it.
dwh