There should be a call-back to the registration to obtain the size of a resource (for display in the list view of the resources tool). If method returns null, byte count should be used. An example of the need for this is the CitationList, where the size is always 36 bytes, the size of the id of the citation-collection. A better "size" for a citation List would be the number of citations. Also, for a video or audio file, the size might be the playing time instead of the byte-count.