Many authors don't bother with chapters any more. Personally, I do prefer having chapters as it breaks the narrative up. You can have scene breaks within chapters if the narratives are linked or they are happening at the same time (so in VydorScope's example above, if they were both attacked at the same time, it can all be in one chapter, intercut between each).
As for length, a chapter, in my view, is only as long as it needs to be
