There;s this kind of "dead zone" where it is hard to sell a piece, ranging from 10,000 to 70,000 words. Novelettes run 7500 to 17,500 (I've managed to sell three of these) Novellas are technically from 17,500 to 40,000 and anything above 40,000 a novel. That said, in the SF/F/H, it's next to impossible to sell anything under 90,000 as a novel these days. In YA you can get away with as little as 70,000.

Middle school books are the best markets for long novellas and short novels.