Your C++ shim control should have a code like this, to resize the hosted
.NET usercontrol (m_hwndUsercontrol):

// When the shim is resized, resize the hosted user control.
STDMETHOD(OnSize)(UINT, WPARAM, LPARAM lParam, BOOL& bHandled)
{
 if (m_hwndUsercontrol)
  ::MoveWindow(m_hwndUsercontrol, 0, 0, LOWORD(lParam), HIWORD(lParam),
true);

 bHandled = true;
 return 0;
}

and your .NET usercontrol should use the Resize event (no need to override
OnResize). In this event, the properties Width and Height of the usercontrol
already have the new values, you don´t need its parent...