The standard in the book world is that anything less than 200 pages is considered a short story or novella, and anything above that is considered a novel. So we try to abide to that general rule.
However it can be a little bit tricky. As even if we exclude large print books, the page count for books can be different due to font used, spacing, version of book. Word count would make more sense (50k and above is a novel, 20k-50k is a novella, and below 20k is a short story) but a lot more difficult to calculate as that information isn't easily accessible.
To add to this - the book publishing world changed in the 80s. Many books prior to that decade would run under 200 pages, but were considered a novel. For a variety of reasons, that changed.
So the entire short story/novella situation is a work in progress. If you spot any books that you feel should be tagged as a short story/novella, let us know. Similar if you see one tagged that you don't believe should be.