Something I always wondered about was how long stories had to be for various classifications. Ran across an article that discussed story length and included the following numbers from the Science Fiction Writers of America:

Short Story - Up to 7,500 words

Novelette:7,500 to 17,500 words

Novella: 17,500 to 40,000 words

Novel: 40,000 words +

Determining page count for an e-book seems to vary all over the place, but the most common numbers appear to be 250 or 300 words for each page. For ease of calculation, when I estimate the length of an e-book, I will use 250 words per page as that seems to be fairly accurate.