I've written tweets of ten words, blog posts of fifty to a thousand, articles between 300 and 10,000 words, and books between 20,000 words and 250,000 words. Different genres, different audiences, topics and approaches require different lengths - which isn't even length really. It is time. Write as long as you need the reader's time to tell the story. In regard to books, and in consultation with your editor and according to your contract, you write the book the length it needs to be so when you're done you feel done, with no unanswered questions. Just don't drop a surprise at the end, and dump a book way long or way short on an editor expecting the opposite.
Much the same with chapters - 2,000 words, 15,000 words - write them, as long as they need to be to feel complete and unified. NEVER write a chapter to length just because you're stuck on a number.
The only real regret I have about any book I've written is when I've compromised according to length. As the late great David Halberstam once told a friend of mine "F-'em. It's your book. Your name is on the cover."
I think of chapter breaks like big breaths, where you feel the need to pause, inhale, ponder and move on - and you have to be a reader here, as well as a writer. Be sensitive to when natural transitions occur - an event comes to a close, a conclusion is reached, a character experiences some kind of defining moment, there is a moment of quiet before action, or action before quiet, some contraction in the narrative. Much of it is just learning to listen to your own work.
It helps, when ending a chapter, to find a way to lift it off the page a bit, and cause the reader to reflect a little, just like the end of a long story or magazine piece, where the story turns back on itself a bit, or the way a piece of music echoes earlier themes. Again, if you are just breaking off for the sake of breaking off, don't. And see if a lead for the following chapter comes easily. If it does, you're breaking it at the right place. But if you neither have an end, or a lead, then you simply might not be at the end of the chapter yet, or have already rushed past it. Trust me, it gets easier the more you do it.
It sounds simplistic, but it really helps sometime to scattershoot through your library just reading leads and ends to chapters, or magazine pieces - can help to brainstorm your own. You'll also realize that some writers you may like a great deal use the same strategies over and over. Nothing wrong with that, if it works, but I must admit that ever since I did that to a writer who I had always admired and realized that nearly every story ended with a similar sensory impression, my admiration dropped just a little. So don't abandon your change up and throw fastballs every time.
And use you outline as that - an outline. Maybe I'm the outlier, but I've never worried for a second about abandoning the outline as I write, as long as I make sure I cover the same territory. For the writer the writing process is also a learning process - no matter how much I think I know beforehand, I don't make the connections until the act of writing takes place, and that can cause me to recast the rest of the book entirely. One of the most lasting things I ever wrote came about when I was in the process of telling a small story that I expected to write over quickly, but found first one question that I didn't have an answer to, then another, then another, and all of a sudden not only did I have an entire new chapter, but the info in that chapter informed the remainder of the book and provided a entire logic that wasn't there when I started writing, and that I didn't know was there in my research.
Another reason why you do this.