I suspect the answers may vary among genres. But can anyone give me a rule of thumb for the numbers of words that involve “novel” range, as opposed to novella, short story, or epigram? (Or on the upper end, what’s past “novel?” James Michener?)
I don’t think one volume of work could transcend a title of “novel”: you’d have to have more than one volume for that. Even if, physically, you can’t fit it into one book if it was meant to be one holistic work, it’s still a novel if it’s in novel form (for instance, “Lord of the Rings”, unless you mean to divide it into 6 books as originally intended.)
I don’t know if there’s a meaningful category of “novella”, it’s either a short novel or a long short story. If it’s 50-100 pages and it still take place over a short period/distance, with little in the way of development*, it’s a long short story. Whereas if it’s longer in duration with more development, it’s a short novel.
*both plot and character.
My understanding was 70000 words was the lower level, particularly if you’re not an established writer.
I’d say anything beyond 50 pages and not much more than 120 is a novella. A novel goes on up from there, though if you’re going to write one of those 1400 pages epics you should probably get an editor to reign you in (Ayn Rand).
Michener wasn’t so much a novelest as an executioner of English grammar who wrote about whatever topic interested him on a particular day, weaving it with something less than deftness into the skeletal framework of something resembling a plot.
Stranger
The way I have understood it is whether it passes the Great Gatsby test - I think GG is about 50,000 words, but gets the job done in terms of telling a story with a beginning, middle, end and with Big Ideas to boot.
So, to me, there is both a length and a structure/intent component.
Where does Heart of Darkness fall? It is considered a short story but, length wise, seems more like a short novel.
I’m not sure how to equate “pages” with words, especially since different editions can have different page counts. Isn’t “words” the standard counter for story length?
A novella, even! It was first published in book form in Youth, which combined 3 such novellas - Youth, Heart and the End of the Tether. (I know - I own the first edition! 
)…
I think novella is meant to represent “shorter in length than a novel, but intended to function the same way as a novel - with a story arc, etc.” vs. a short story, which doesn’t need to accomplish all the things a novel accomplishes…
If you like. However, I’m not going to sit and count all the words in a work to figure out whether it’s a novel or novella. 
Seriously, I don’t know that there’s any hard and fast criteria, particularly between a long short story and a short novella, or between a long novella and a novel. I’d tend to break them down more by their thematic and structural complexity; a short story deals with one idea, a few characters with minimal development, and a limited range of subjective time, with very tight constuction, often with a quick suprise ending. A novella has slightly more complexity and considerably more character development, but lacks the depth of substantial changes, and has a looser structure; it might have a twist ending like a short story but there has to be more to it. A novel sweeps across many themes, a greater range of time, and allows more range to explore ideas. Twist endings don’t usually work well in novels, because they defeat the purpose of having spent tens of thousands of words to create a fictional world for the reader. (Hannibal is an egregious example of this.)
That being said, I think it’s all pretty interpretive. The Grifters is arguably a novel in length and complexity, but a novella in how it plays into the gut-shot of an ending. And some novels don’t really have a plot structure at all; Joseph Heller’s Something Happened, or pretty much anything written by Kurt Vonnegut after Cat’s Cradle lack any real narrative coherence or a thematic structure, so where do they fall in?
I suppose I’d say that I can’t describe a novel, but I know it when I see it…well, sometimes.
Stranger
Off-topic, but I’d seriously disagree with this. Many types of twist endings are very appropriate for novels. The unreliable narrator, best exemplified by Agatha Christie’s The Murder of Roger Ackroyd, is best served up by a novel as opposed to a short story. Dashiell Hammett’s puzzle-piece novels involved ending I’d characterize as “twists,” as well.
Officially, for science fiction, a novel is anything over 40,000 words.
Short story: <7,500
Novelette: 7500-17,500
Novella: 17,500-40,000
Novel: >40K
From a marketing point of view, however, publishers don’t like novels under 70K words, especially when it comes to first novels.
Yes, but in publishing a “word” is a specific value: 6 characters, and is tied in with the amount of vertical space used. You count the number of characters in a line, divide by 6, and multiply by the number of lines in the work. A one-word paragraph takes up as much space as a full line in a paragraph, so the count needs to factor that in. Your word count on your computer is up to 20% short. See this article.
You can use the word count method to figure out the length of a book. Count the number of word in a mid-paragraph line (i.e., not the first or last lines.) Divide by 6. Multiply by the number of lines on a page. Multiply by the number of pages (corrected for any half pages).
There’s no good answer to this because the question is too broad. It’s like asking "how large is a boat?’
What purpose are you striving for in the question? The answers vary considerably.
E.g., if you want to submit a manuscript, find the publisher’s guidelines. What the publisher says is a novel is a novel. Unless the marketing department insists on calling a novella a novel because novellas don’t sell. Or the marketing department might insist on taking Lord of the Rings, which is a single novel, and publishing it in three parts so that everybody starts calling it a trilogy, even though it is a single novel.
Are you asking whether novels are gotten longer or shorter over the years? Whether different genres aim at different reading experiences? Whether printing technology has size limits? What a “good” size for a novel is aesthetically?
Whatever, please be a lot more specific.