One of the things I've discovered as I evolve in my writing style is that I'm pretty comfortable doing chapters in the 4-5k range. That being said, I also like to challenge myself. I wrote two back to back stories where I kept the chapters in 5k territory.
My biggest challenge so far was last year's flash fiction. Limiting a story word count to 1k is really hard when you're used to being able to ramble on. It was a fun learning experience and it turned out well. It's one of my favorites.
Overall, it's whatever you're comfortable with.