nvm, I’ve answered my won question with the following code:
var veryLongString = "insert very long text here";
var scrollPosition : Vector2 = Vector2.zero;
scrollPosition = GUILayout.BeginScrollView(scrollPosition, GUILayout.MaxHeight(300), GUILayout.ExpandHeight (false));
GUILayout.Label (veryLongString, GUILayout.ExpandHeight (true));
GUILayout.EndScrollView();
This will create an area with 300 height. Scroll bar will appear if the text is too long.