900 to 3500 is about the range I usually get, and I think that's perfectly reasonable. There's nothing wrong with chapters of different length (or short ones).

That said, having chapters of similar length in pairs sounds like a interesting feature in a novel. If you can make it work. I wouldn't worry too much at this stage, though. Keep track of your words if you want to, but don't let them get in the way of your work.