So far, I've been averaging about 12 pages (6000 words) per chapter. It's all single spaced in MS Word.
Does this sound too long for chapter length?
As you said, there is no rule. There are a few guidelines, depending on what genre you're writing. For me, as a romance writer, I tend to stick to about 12-15 double spaced pages. That's close to "average". If you're writing fantasy or sf or mystery or something, that number could change.
Also, I was wondering what the ratio of word processor page to published book page is. Like, does one MS Word page equal one published book page? I'm a little confused when agents or publishers ask for a 5 page minimum submission or something like that. Does that mean five pages of Word, or five equivelant book pages?
SMF (Standard Manuscript Format) is generally considered to be Courier or Courier New 12pt font, double spaced, with 1" margins all around. If that's what you're using, you get approximately 250 words/page, which is book equivalent. This would mean that, yes, they're asking for 5 pages in your submission.
The important thing, of course, is to check the publisher or agent's guidelines. They may want some other kind of format, or go by computer word count, etc, and that could change what you turn in.
Hope that helps.