I often check on amazon the book's paperback version page count. Also, if the font is standard, I check the number of pages on my Sony reader. An average length novel (300-400 pages in paperback) will be around 600 pages on my Sony. Longer novels are around 1000-1500 pages long. The Pillars of the Earth is 2258 pages long on my reader.
I googled about the word count in books and found out that amazon sometimes has a text stats page (search for "text stats" on the book page).
Click here for an example.
Oh and the number of characters would be even more accurate than the number of words to describe a book's length (what if someone decides to write a book using only lengthy words?!). Sorry, I'm an engineer

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