When I first started writing Red Mercury I just got on with it, trying to write a few hundred words every day, keeping the story moving along. However, after a few weeks I started to worry about how long it 'needed' to be. Was this going to be a short story? A novella? A novel? What length is a book?
Looking online, the general consensus seems to be anything less than twenty thousand words or so is a short story. Anything above that but less than forty thousand words is a novella. Anything more than that is a novel. As a result of this info, I became slightly obsessed with hitting 40k and writing a 'proper book'.
As it was, my first draft came in at just over forty thousand words, and I was all pleased with myself. However, my ruthless editor (ie. wife) has a great eye for padding and pointless paragraphs, and through judicious use of the red pen, reduced that first draft by a full 10%. She decimated it. Literally.
There is no doubt in my mind that the story is much better as a result of this pruning - punchier, sharper, more active, more exciting. But now it's not a 'proper novel'.
I've decided to refer to is a 'novella', even though I don't really like the word. I think it's a bit fey - especially for an action story. But novella it is. The last thing I want when I finally publish is for anyone to complain that it's shorter than they expected.
Of course, over time, the requirement for such a distinction is going to completely evaporate. That 40k number was originally driven by the minimum page count which old hardback binding technology could handle. Over hundreds of years, this technological requirement has created an expectation in people's heads about how long a book needs to be. But in the world of e-books all these old standards will eventually wither away. We're going to end up with short stories, long stories, short novels, long novels, epic trilogies - and their 'length' will be essentially invisible, experienced only as the reader reads, rather than in measureable inches of dead tree visible on a bookstore shelf.
My favourite story ever is one that Gibbo tells about another mate, Irish Dave. I doubt the word count on that one goes above a couple of hundred. Not one of them is safe for online publication.