From what I understand 80,000-100,000+ words is the norm for a novel. Though there will be those that say it is a novel as long as it is above 50,000 words. So make of that what you will, since 50k is a novella, by some measure of standard. Which gives you plenty of freedom to make it as long or as short as you like. The story is as long as you want it to be (even if you don't plan on it having an ending).
as long as it needs to be - troll is right for a thriller or adventure but it varies a lot - romances are I think typically about 60k If you want a trad deal I'd say no longer than 120k as a maximum, for self pub just do what you want
Depends on a variety of things like genre, whether or not you want to self-publish or find a publisher, and the target audience's age. I'd say 50k and up though is a novel because 50k really isn't a novella anymore imo. I'll put my frame at the extremes and say anywhere from 50k to 150k. That's bare bones all the way to an epic, though even those could be longer, up to 250k, that's not a good place to start as a new writer probably. A few examples: Fantasy/sci-fi are often the longest(100k+ at times), but romances and mysteries can be on the shorter end(can fall as low as the 50k-70k range). Middle grade novels are often short than YA which is shorter than adult lit. I don't know the exact number on these, but they'll also shift a bit from genre to genre. E-book and paper print may be held to different standards. Ultimately the sweet spot is likely around 70k-90k.
