When planning iteration lengths, don't get too caught up in sticking to a standard timebox. Yes, doing so DOES make it easier to build your release plan; however, the goal of agile management approaches is to be FLEXIBLE and adapt to the specific needs of the situation you find yourself in. In some cases, user stories cannot get broken down enough to fit neatly into your standard iteration length. There is nothing wrong in these cases with creating a specific, longer iteration to deal with such a user story, then switching back to the standard iteration length for later iterations. The main focus should be the delivery of business value in each iteration -- if it requires a few extra days (or even a week or two) to deliver such value in one of your iterations, then feel free to adapt the iteration length to accommodate the situation.

Beware, however, that stretching the iteration length too long -- more than just a few days -- will have a distorting effect on metrics such as velocity, and you may want to hold off adjusting your release plan following such an iteration to avoid replanning based on the velocity achieved in a unique circumstance.