Length is a terribly subjective topic, and can depend largely on the style in which an author chooses to write and the feeling they're going for. In regards to word count (which I find more accurate than file sizes, for a variety of reasons mentioned above), some of my stories have chapters averaging at 5k per, while others float between 2k to 3k words.
What determines chapter length for me is a question I ask myself at the end of every scene I've written thus far: What's happened in this chapter to advance the plot? If I answer to myself with "This, and this, and this," I consider that the cut-off point. Long chapters annoy me as an author because it means I've got to do a lot more work if I decide, mid-writing, that I want to go back and verify a particular plot-point. Besides, I've noticed that beyond roughly 4k words or so, my writing slows down significantly. Just a weird mental block, I guess.
As a reader, I don't necessarily care about length, I care about plot. I've been annoyed by 15k words of absolutely nothing happening and pleased by 2.5k words of plot advancement.