I know people who will only go for long books. I'm not sure if this is a way to get their money's worth or because they feel more drawn into the story if it's longer. Others like short books because they don't read much or don't have much time and the sight of a big fat brick is a bit overwhelming.
I don't have a set preference but I usually go for average sized books (about 350 pages) because I feel I can read two books in the time it would take me to read an 800 page book. But this isn't always the case, because there are books that keep you so hooked you'll finish them in no time, whereas others you'll struggle to get through it and it will take longer. Also, there is the patience factor. I'm quite impatient and I want to know the end as quickly as possible, so really long books will have me racing through the pages to get to the (hopefully) happy ending.
With blog posts I've found I leave the longer ones for later when I've got more time to read them if it's something that interests me. However I'll read a short post right there on the spot.
What do you prefer? Do you like short or long books? Do you think about it before buying a book?