CHOOSING SIZE OF THE LAMPSHADE
The height and width of the lampshade and the lamp base should not be the same. You can calculate the height by dividing the lamp into three thirds, where the lamp base is 2/3 and the screen is 1/3. This means that if your lamp base is 40 cm, the shade should be half as big, which is 20 cm in this case.
The width of the screen should be twice as large as the lamp base. If the lamp base is 10 cm wide, the shade should be around 20 cm wide, 10 cm on each side when it is placed on the lamp base.