Retrieves the formatting rectangle of a control
#include <GuiRichEdit.au3>
_GUICtrlRichEdit_GetRECT ( $hWnd )
| $hWnd | Handle to the control | 
| Success: | an array consisting of x and y coordinates. $aArray[0] = Left $aArray[1] = Top $aArray[2] = Right $aArray[3] = Bottom | 
| Failure: | sets the flag to non-zero. | 
| @error: | 101 - $hWnd is not a handle | 
The formatting rectangle is the area in which text is drawn, part of which may not be visible.
All returned values are in dialog units referenced to the control.
Per MSDN, the values returned by this function may not be exactly what may be set by _GUICtrlRichEdit_SetRECT().
Search EM_GETRECT in MSDN Library.
#include <GUIConstantsEx.au3>
#include <GuiRichEdit.au3>
#include <WindowsStylesConstants.au3>
Global $g_idLbl_Msg, $g_hRichEdit
Example()
Func Example()
    Local $hGui, $iMsg, $idBtn_Next, $iStep = 0
    $hGui = GUICreate("Example (" & StringTrimRight(@ScriptName, StringLen(".exe")) & ")", 320, 350, -1, -1)
    $g_hRichEdit = _GUICtrlRichEdit_Create($hGui, "", 10, 10, 300, 220, _
            BitOR($ES_MULTILINE, $WS_VSCROLL, $ES_AUTOVSCROLL))
    $g_idLbl_Msg = GUICtrlCreateLabel("", 10, 235, 300, 60)
    $idBtn_Next = GUICtrlCreateButton("Next", 270, 310, 40, 30)
    GUISetState(@SW_SHOW)
    _GUICtrlRichEdit_SetText($g_hRichEdit, "First paragraph")
    _GUICtrlRichEdit_AppendText($g_hRichEdit, @CRLF & "Second paragraph")
    While True
        $iMsg = GUIGetMsg()
        Select
            Case $iMsg = $GUI_EVENT_CLOSE
                _GUICtrlRichEdit_Destroy($g_hRichEdit) ; needed unless script crashes
                ; GUIDelete()   ; is OK too
                Exit
            Case $iMsg = $idBtn_Next
                $iStep += 1
                Switch $iStep
                    Case 1
                        Report("1. Default settings ")
                    Case 2
                        _GUICtrlRichEdit_SetRECT($g_hRichEdit, 10, 10, 100, 100)
                        Report("2. Settings ")
                    Case 3
                        _GUICtrlRichEdit_SetRECT($g_hRichEdit)
                        Report("3. ReSettings to default")
                        GUICtrlSetState($idBtn_Next, $GUI_DISABLE)
                EndSwitch
        EndSelect
    WEnd
EndFunc   ;==>Example
Func Report($sMsg)
    Local $aRect = _GUICtrlRichEdit_GetRECT($g_hRichEdit)
    $sMsg = $sMsg & @CRLF & @CRLF & "Left=" & $aRect[0] & " Top=" & $aRect[1] & " Right=" & $aRect[2] & " Bottom=" & $aRect[3]
    GUICtrlSetData($g_idLbl_Msg, $sMsg)
EndFunc   ;==>Report