Hi,
Initialize is the very first event fired in the component life cycle. As good as a constructor of a class.
In this event you get no values pertaining to the component like the actual height, width, etc.
CreationComplete is the event which gets fired at the fag end of the component life cycle after the whole component is completely created along with its childrens if any. In this event you will get values pertaining to the component like the actual height, width, etc as the component is already created here.
Note: Any particular tag on the MXML file is nothing but a component.
Hope this helps you and suffices your need.
Regards,
Amol.