You cannot keep maximized windows from changing when you open a new window. You can, however, minimize the size change keeping the original windows at the largest possible size to fit inside the application window that appears when you create the new window.
Use Select window to select the original window
Then use adjust windows to set the window dimensions back out to the largest possible sizes. get ( Screenheight ) and get ( ScreenWidth ) can be used to get the maximum height and width of your original window.
You can find examples of this technique in the scripts that open new windows in the Known Bugs List database.