Resizing Controls in MFC

A window receives WM_SIZE message (which is processed by OnSize handler in MFC) immediately after it was resized, so CEdit::OnSize is not what you are looking for.

You should add OnSize handler in your frame class and inside this handler as Rob pointed out you'll get width and height of the client area of your frame, then you should add the code which adjusts size and position of your control.

Something like this

void MyFrame::OnSize(UINT nType, int w, int h)
{
    // w and h parameters are new width and height of your frame
    // suppose you have member variable CEdit myEdit which you need to resize/move
    myEdit.MoveWindow(w/5, h/5, w/2, h/2);
}

When your frame receives an OnSize message it will give you the new width and height - you can simply call the CEdit SetWindowPos method passing it these values.

Assume CMyPane is your splitter pane and it contains a CEdit you created in OnCreate called m_wndEdit:

void CMyPane::OnSize(UINT nType, int cx, int cy)
{
    m_wndEdit.SetWindowPos(NULL, 0, 0, cx, cy, SWP_NOMOVE | SWP_NOACTIVATE | SWP_NOZORDER);
}

GetDlgItem(IDC_your_slidebar)->SetWindowPos(...) // actually you can move ,resize...etc