Help from Just Shades - How to measure your shade
A good rule of thumb is to choose a lamp shade that's two-thirds the height of the base. That proportion assures that the lamp won't look top- or bottom-heavy. You also want to be sure that the lamp shade doesn't expose any of the hardware underneath, such as the harp (the wire frame around the light bulb).
Size in inches:
(top x bottom x height)
The widest part of the lamp shade should be at least a half-inch wider on each side than the widest point in the base. Also consider the location of the lamp: if it's going on a nightstand, you don't want to crash into it every time you get in or out of bed. Same thing for a lamp on a hall table — don't let it stick out so far that you bang into it when you walk by. Seventeen to 20 inches is standard for a large lamp shade; anything over 20 inches risks looking too big.
Candlestick lamps can support just about any shape of shade, but if you have a base that's round, it will look best with a round lamp shade. Likewise, a base that's square or angular generally looks best with a square lamp shade. The exception: If you're putting a square-based lamp on a round table, it's okay to echo the table top by using a round shade.