To choose the ideal height (length) for your lamp shade, first measure the height of your lamp base, not including the lamp harp.  Please note: if your lamp base does not take a harp, Alluminare lamp shades will not fit!  In general, we have found that the best length for a lamp shade is about 2/3 the height of the base.   For example:

15-inch tall base: 10-inch tall shade

18-inch tall base: 12-inch tall shade

21-inch tall base: 14-inch tall shade

Make sure, however, that you choose a shade length at least 2 inches longer than your lamp harp, so that the bulb, socket and other non-decorative base hardware is fully concealed from view.