I'm working with writer Nancy Rockwell who writes TheBiteinTheApple blog, which as she describes it, a series of essays based on re-imaginative theology that works at the intersection of literature, legend, belief, spirituality and theology (it's a busy intersection).
It's a wonderful blog that receives about 15,000 hits each month.
At any rate, she asked how long a single chapter of a book should be and how long a book should be. The short answer is: Long enough to tell your story.
There are no rules, or at least I don't like rules if they get in the way of the story I'm trying to tell.
As an example, in her latest memoirs, Joan Didion uses chapters that are two or three paragraphs, but they do the job of carrying her story.
So when thinking about what a chapter needs to be, it needs to be strong enough to carry its part of the story, no more no less.