I think there is a way to set it in the registry. I remember reading that to start off with windows remembers your preferred size of window after a 100 or so times its opened, then it moves onnto only changing the size it opens up with after 1000 or so.
Annoys me aswell, mainly becasue the window opens nearly maximized but not :/
Ill go see if i can fine it seeing as im bored ( at college )
edit: Perhaps this will help? i dont know if its the right thing but anyway,
here