What if you copy your richtext item to newly created document and check size of whole document? I think that is quite simple and easy solution (and will be not a problem with access as we do not need to save document). Then you do not need to care about different kind of data into richtext field.
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Set rtitem = doc.Getfirstitem("body")
Dim newdoc As New NotesDocument(db)
Call rtitem.Copyitemtodocument(newdoc, "body")
print newdoc.Size