The measurements that you will need to select a shade of the right size are the top and bottom width of the shade and the height of the shade without the lamp. You can use a regular ruler to measure these as it doesn’t have to be really accurate. Because most of the shades have a slanting side, you have to make sure that you measure the vertical height of the shade and not the side length. Apart from the measurements of the shade itself, to compare the sizes of both. For this, you have to measure the base width of the lamp and its total height. When shopping, you can compare both when you are selecting a shade.