Belini,
 
	This is how I do it:
 
#include <GUIConstantsEx.au3>
#include <WindowsConstants.au3>
; Set resize mode for controls
Opt("GUIResizeMode", $GUI_DOCKAUTO)
Global $iGUIInitSize = 500
$hGUI = GUICreate("Test", $iGUIInitSize, 500, -1, -1, BitOR($WS_SIZEBOX, $WS_SYSMENU))
$cButton = GUICtrlCreateButton("Resizing text", 10, 10, 80, 30)
$iLast_Control = GUICtrlCreateDummy()
GUISetState()
GUIRegisterMsg($WM_SIZE, "_WM_SIZE")
While 1
	Switch GUIGetMsg()
		Case $GUI_EVENT_CLOSE
			Exit
	EndSwitch
WEnd
Func _WM_SIZE($hWnd, $iMsg, $wParam, $lParam)
	#forceref $iMsg, $wParam, $lParam
	If $hWnd = $hGUI Then
		; Calculate required font size
		Local $aGUI_Size = WinGetClientSize($hGUI)
		$iFontSize = Int(2 * (.25 + (8 * $aGUI_Size[0] / $iGUIInitSize))) / 2
		; Reset font size for all controls on main GUI
		For $i = 0 To $iLast_Control
			GUICtrlSetFont($i, $iFontSize)
		Next
	EndIf
EndFunc   ;==>_WM_SIZE
	M23