With Q3 2013 we released a couple of virtual methods(CalculateShapeBounds, CalculateContentBounds and CalculateMinShapeBounds) that you can override and change the default container behavior (auto resize, minimum size etc.).
I've attached a sample project with very simple implementation of fixed container - turned off the container resizing and overrode the CalculateContentBounds so that it doesn't auto resize on item drop.
I hope I was able to help you and if you have more questions please feel free to ask.
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