Given the Clute/Grant Encyclopedia of Fantasy can't seem to settle on one in 1.5 million words and 6,000 entries (and their own three-page definition of 'FANTASY' includes numerous caveats and exceptions), I don't really see anyone else coming up with one that works satisfyingly either, especially when you factor in people arguing if the magic realists should be on there or not: Clute & Grant lean towards inclusion of works with undeniable fantastical elements, but seem divided over works where the characters' own delusions and fantasies play a major role in proceedings.

You then have something like Christopher Priest's THE AFFIRMATION, where the entire fantasy story takes place in the main character's head, but the fantasy story is incredibly vivid, takes place in a richly detailed secondary world and events within that world are given tremendous resonance and importance. Priest dodges the bullet on that one by introducing enough doubt into the finale to make it appear that the main character is actually a native of the secondary world and has dreamed/fantasised our world into existence instead, which would make it indisputably of the fantasy genre.