Version: 2017.1
public bool maximized ;

Description

Is this window maximized.

Set this value to maximize / unmaximize a window. If the window is undocked, this value will always be false and setting it will have no effect.

class Maximizer extends EditorWindow {
    function OnGUI() {
        maximized = GUILayout.Toggle(maximized, "Maximize window");
    }
}