You can use the following diagram constants:
DiagramConstants.MinimumAdornerSize = 5;
DiagramConstants.MinimumShapeSize = 5;
MinimumAdornersize will set the minimum size of the manipulation adorner which you use with mouse to resize the shape. For better user experience it's good to keep these 2 constants with equal values.
However, although they are set to 5 , you will be able to resize the shapes with lower values using the Diagram Settings Pane.
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