File_column: post processing to add or delete thumbnail size

Dear all,

I have been using file_column for sometime now, and have several
pictures which have all been uploaded by file_column. Now, I would
like to add more thumbnail sizes for various different purposes –
and perhaps to even delete the sizes I no longer use. Is there a way
to do this using file_column automatically? Or can someone please
point me out to a script or a hint on writing a script which would
take care of this for me.

Thank you,