Windows 7 should automatically "remember" the current size of a window when it's closed, but may not always do this properly. (For instance, if you open Microsoft Word, then make the window larger or smaller, when you close Word, it should save that setting and open all documents in a window that size until you change it and close again.)
To force Windows to record the size of a window and set it as the default size for that program each time it's opened, simply resize the window to your preferred size, then hold the CTRL key while you click the X to close it. The program should use that size as the default size until changed (by resizing then closing).
From answers.microsoft.com - (discusses Internet Explorer 8 but works for all programs under Windows)
Keywords: Windows, default window size, resize