Peruse many of the lists of the top books of all-time and you will find lots of very long books. War and Peace, Ulysses, and Don Quixote are on seemingly every list, and each of them tops 700 pages. This is incredible, given that undertaking such an epic novel is such a time sink. Because of this conundrum, I have long wondered whether there is a bias favoring long books over shorter ones.
So why do people rate long books so highly? I think it's a case of cognitive dissonance. Readers know full well how long the book will be before they decide to read it. They then spend so much time and energy reading the book that they come to believe it must have been good.
There are other possibilities, such as a selection bias, where people only rate books that they have finished. People may be more likely to stop reading a longer book that they don't like, as opposed to a shorter book that they may simply power through. This strikes me as flawed for two reasons. One, many readers are cursed with the terrible affliction of finishing all books they start, and two, people are liable to form an opinion after reading nary a sentence.
The best evidence for a cognitive dissonance effect in long books may come from movies, where longer flicks are not as disproportionately represented on the "best of" lists. Before watching a movie, you are much less likely to know exactly how much longer the movie will be, and you may become bored goes on and on. But since you didn't know beforehand how long the movie will be, it's somehow not your fault if the movie is too long for your taste.
What conclusions can we draw? My advice is to take opportunity cost into consideration when recommending a book. For every doubling of book length, the value extracted should also double. Unless it's Godel, Escher, Bach: An Eternal Golden Braid, a long book which I recently gave a 5/5 on Shelfari. But that one must have been good... I spent over a month reading it!