When I'm writing I don't think about how long a novel is going to be, I'm just writing, and although I obsessively check my daily out put and moan or cackle gleefully depending on how little/much I've achieved, I don't bother with what the running total is. But despite this I usually come in at about the same amount, about 100,000 words. Most writers I've spoken to say that they're the same, writing without watching the word count but ending up with a fairly consistent word total.
As a general rule - and as we all know, rules are of course made to be broken - a mainstream adult novel is between 80,000 - 110,000 words. You'd expect a saga to be towards the top end of the scale or even beyond it, perhaps 120,000 words. Genre fiction - crime, fantasy, romance - might dip under 80,000. Mills and Boon comes in at 5o,000, although the historicals are 70,000-75,000 words.
A novella is perhaps 40,000 - 60,000, but the truth is that they don't sell unless by well known authors and even then there may be grumblings - every review on Amazon of Ian McEwan's On Chesil Beach mentions the shortness of the book. We expect our books to be a certain number of pages and feel cheated if we don't get them, however gorgeous the prose. If you're determined to write novellas, can I suggest joining two (or three) to make a longer work, preferably one that could be marketed as a novel. Or look into self publishing.
Children's fiction is divided into age bands. Books aimed at the very youngest, the picture book texts and first readers, would come in at a maximum of 1000 words, though could be far fewer. Books for 5-7 year olds are around 1000-6000 words, 7-9 year olds 5,000 - 20,000 words, 9-12 year olds, 15,000 - 60,000 words, 12 upwards, 30,000+ words. A lot depends on the book - obviously the Harry Potter books start at upwards of 100,000 and just get longer, which meant publishers started buying longer children's fiction.
Book length is determined in part by fashion (see Harry Potter) and in part by external costs. In the 1980s bonkbusters and sagas could weigh in at 200,000+ words, but have become shorter as the paper costs have gone up. Our attention span is also believed to be shorter, though I'm not sure if that's really true (see Harry Potter. Again.).
If you finish and you're way over or under these guidelines, you'll either have to cut, add, or get creative - Lord of the Rings was originally published as 3 books; many a novel started as short stories. But overall I think it's best to simply write the best book you can, and worry about length later.