I suggest you take a look at email field and how it does its processing, AFAIK it works now with fields without storage, same logic should work for you.
I opened the email.php file in the plugin but I can't see any information on how to retrieve the filesize of the attachment and to put it into the email body. Does it have to do with $options2['attachment_field'] ? Thank you!
I don't think this information is available in afterstore at all, I think you only option would be to read the file again and extract its size like in upload_file.php onCCK_FieldPrepareStore method.