void CChildView::OnPaint() 
{
	CPaintDC dc(this); // device context for painting
	
	// TODO: Add your message handler code here

	CRect win;
	GetClientRect(&win);
	CString str;
	str.Format(_T("width %d height %d"), win.Width() , win.Height());
	dc.DrawText(str,win,DT_CENTER | DT_SINGLELINE | DT_VCENTER);
	
	// Do not call CWnd::OnPaint() for painting messages
}

You can control the size of the application window by making a few changes to the CREATESTRUCT in the PreCreateWindow method in MainFrm.cpp.

BOOL CMainFrame::PreCreateWindow(CREATESTRUCT& cs)
{
	if( !CFrameWnd::PreCreateWindow(cs) )
		return FALSE;
	// TODO: Modify the Window class or styles here by modifying
	//  the CREATESTRUCT cs


	// specify requested main window size (includes frame)
	cs.cx = 400;
	cs.cy = 400;

	cs.dwExStyle &= ~WS_EX_CLIENTEDGE;
	cs.lpszClass = AfxRegisterWndClass(0);
	return TRUE;
}

Results