I find that length is a book characteristic whose variety keeps interest levels up. Or, put another way, I like to switch between reading shorter stories or books with longer works.
I am currently reading Peter F. Hamilton’s Judas Unchained which is as just about long as its predecessor, Pandora’s Star. Clocking in at 800+ pages, it’s one of those books that take longer to read than your average science fiction book. (Fantasy books, of course, are a different matter. The law requires fantasy books, especially quest fantasies, to be a minimum of 800 pages. Violation of this law results in much finger-pointing and ridicule.)
It may be partly because of my experience with Judas Unchained (more on that when I post my review), but I find myself (as I often do when reading longer works) wanting to read something shorter next time. In fact, I’ve even gone so far as to pull out a few shorter books from the “archives” (which I sometimes refer to as “the boxes that litter my home”). I’m acquiring the taste for the short, sharp shock of, say, Clifford Simak’s The Trouble with Tycho, his novella packaged in an easily-digestible book form. I also have Edgar Rice Burroughs’ Beyond the Farthest Star on tap, a collection of two serialized novels. It’s not that I’m trying to keep my book count high, but I do find that consuming shorter works more frequently means there are more places I can visit with my imagination.