I understand that a single letter, or even a word cannot be copyrighted, because they are too short to be a unique work. So how big does a work need to be to qualify?
It must be an “original work of authorship”. For a court to consider it such it must be complex enough that it is likely to be “original”.
Length is not the sole criteria, compositional effort is relevant - short poetry is more likely to be “original” than short prose, however, even short prose can qualify if the originality is there. For example: “For sale, Baby shoes, Never worn.” erroneously credited to Hemingway almost certainly qualifies.