RadChart performs the layout calculations only once. RadChart exposes two events - BeforeLayout
former is fired before the layout is calculated, while the latter is
fired after that and before the actual image rendering occurs. That said, you can retrieve the plot area size in the PrePaint
event handler. However, you will not be able to take advantage of this size in scenarios related to RadChart layout, as it has already been calculated and any updates in chart elements size and position will not be applied.
the Telerik team