The behavior of Control.GetPreferredSize(System.Drawing.Size) differs by control. The Control.LayoutEngine might be unable to assign the returned size to the control. You can return a size larger than the constraints indicated in the proposedSize parameter, but proposedSize should decrease as the constraint decreases. For example, GetPreferredSize(new Size(100, 0)) should not be wider than GetPreferredSize(new Size(200, 0)). The exception is a proposedSize of 0 or Size.Empty, which are defined as unconstrained.