I am unsure if I understand your question utterly. The RadChart (being a SL Control) can not be larger than the plugin it resides in. So if the plugin does not take the whole space of the browser, the chart also can't. If you place the chart in the most top container of your app (LayoutRoot of the main window, may be) then it can be possible for the chart to stretch to its fullest. Or perhaps you need to use some popup functionality.
Note that the RadChart is, after all, a Control. So if you can achieve what you are after with any other control then it should not be very different to achieve the same it with RadChart.
the Telerik team