For me a chapter has its own arc, so that at the end the characters are in a noticeably different place - emotionally, practically - from the beginning. It's a unit of the story, if you like Which is why it makes perfect sense to say 'as long as it needs to be', because it's the sense of the shape and pace of the story that defines 'need'. It might be one big scene, or a series of shorter scenes which add up to one big change. The risk of thinking of them in that way is that the end of the chapter is so concluding that the reader doesn't feel the need to go on - the opposite of a cliffhanger (planesitter?) - if you like, so you still need to make sure the seeds of the next conflict/drama/action are visibly germinating, even as the arc of the chapter is curving down towards its end.
Having said 'as long as it needs to be' I find that my chapters do come out very roughly the same length, plus or minus about 15%, at a guess.
Having realised that to be true for me, I do keep an eye on the word count as a double-check that I'm not over- or under-writing compared to earlier chapters.
FWIW, my chapters in TMOL vary between 10,000 and 14,000 words, but because it's a parallel narrative, each one has at least 3 or 4 sections of the different narrative. And the whole beast divides into 4 parts, so there are larger and smaller structural units, as well as chapters.
You really wanted to know all that, didn't you!
Emma