I cannot speak for calibre, but I know that the word count demarcations for books have always been a bit contentious for the book community as a whole (readers, authors, publishers, etc).
I intend to do something similar within my own database. However, I'm just adding the size terms as normal tags, not a new custom column. While helpful, I don't believe this information to be so critical that it must be handled as a distinct and discrete bit of data.
However, to play Devil's Advocate
, I could see perhaps a derived column which would take the page count, perform a look-up on the size per your table and then display "Long Novel (927 pgs)" into the display, or similar, for each book record.
Now I'm not sure how that could be implemented exactly.... but apparently theducks has!