"How Many Pages Make a Novel" now has a comment thread several years long, and it's actually awkward for me to track through the thread to try to answer the latest questions. So let me try to re-state some basic points, and to answer some recent questions in the process.
A short-short story is anywhere from 250 to maybe 1500 words. A short story is 1500 to 10,000 words. A novelette could be 10,000 to 30,000 words. A novella is 30,000 to 55,000-60,000 words, and anything longer is a novel. However—
These are all terms from the early part of the 20th century, when popular magazines ran a lot of fiction. A few writers made very comfortable livings selling short stories and novelettes to women's magazines, men's magazines, general-interest magazines like Saturday Evening Post, and even newspapers.
One men's magazine built itself around not running fiction: it was called True, and every item in it was nonfiction. (I loved it, but I also loved Bluebook, a men's magazine that ran many early Robert Heinlein novels in serial form. This was so long ago that "men's magazines" didn't have pictures of naked girls.)
For a long time, the short-story and novelette market has been a ghost of its former self. The only "popular" markets are a handful of fantasy, SF, and mystery magazines. University-funded "little" magazines will sometimes run a short story or two, but they pay little or nothing. High-end magazines like The New Yorker and Harper's will run short fiction, but usually only by established authors.
The chief markets for a new writer, therefore, are genre novels for the mass market (SF, fantasy, thrillers, romances, etc.) or for the young-adult market. YA novels are usually shorter (20,000 to 40,000 words—but then there's Harry Potter!). Most genre novels for adults start at 60,000 words and work their way up to fantasy series in which each novel is maybe 125,000-250,000 words long...and the series goes on for as many titles as readers will buy.
Most publishers' websites, if they have a page of advice for contributors, indicate the general length of the books they publish. But you can also get a sense of that by simply counting the words on two or three random pages of a book, averaging the numbers, and multiplying by the number of pages.
But bear in mind that layout and design can influence the size of a book. Sometimes a short novel can be "bulked up" with wide spaces between lines of large text. This gives purchasers the illusion that they've bought a lot of reading for the price of the book.
I don't want to discourage anyone from writing at whatever length they like. Short stories can be great reading, and also great training for writers aiming at novels. Two or three novellas, published together, can be quite marketable (preferably, however, if the author's already well known).
In general, I'd advise writing long. It's easier to cut than to add material. So if you want to sell a romance and you know the length is going to be 55,000 words, shoot for 65,000...and then go back through the manuscript and cut the 10,000 weakest words.