in the case of a floating panel you can hook into the parent RadPaneGroup:
RadPaneGroup paneGroup = (RadPaneGroup)this.pane.Parent;
paneGroup.SizeChanged += pane_SizeChanged;
void pane_SizeChanged(object sender, System.Windows.SizeChangedEventArgs e)
{
this.UpdateSize(e.NewSize.Width, e.NewSize.Height);
}