250,000 pages is unrealistically ambitious (6)
I believe the answer is:
dreams
I'm afraid I can't even guess how this answer relates to the clue.
Can you help me to learn more?
(Other definitions for dreams that I've seen before include "'''We are such stuff as ... are made on'' The Tempest (6)'" , "Thoughts and visions during sleep" , "Fantasies; fantasises" , "Fantasy ambitions" , "Reveries or cherished aspirations" .)