Thanks for tips guys! Was useful for me.
Also I've did some changes, and now IP address input box changes it position proportionally to window size. May be it will be useful for somebody too..
$GUIWidth = 401
$GUIHeigth = 528
$MainForm = GUICreate("Cisco Access Port Finder", $GUIWidth, $GUIHeigth, 661, 215, BitOR($GUI_SS_DEFAULT_GUI,$WS_MAXIMIZEBOX,$WS_SIZEBOX,$WS_TABSTOP,$DS_MODALFRAME,$DS_SETFOREGROUND))
$IPAddress1 = _GUICtrlIpAddress_Create($MainForm, 225, 140, 165, 22)
_GUICtrlIpAddress_Set($IPAddress1, "0.0.0.0")
GUIRegisterMsg($WM_SIZE, "setippos")
Func setippos($hWnd,$iMsg,$Wparam,$lparam)
Local $clientWth = BitAND($lParam, 0xFFFF)
Local $clientHt = BitShift($lparam,16)
WinMove($IPAddress1, "", Int(225*$clientWth/$GUIWidth), Int(140*$clientHt/$GUIHeigth))
EndFunc ;==>setippos