First off Carolyn, great lamp! We're totally jealous and while the shade you have on it is ok, that lamp could really sing with the right one. We'll let everyone chime in, but here are general rules of thumb for picking lampshades:
According to this guide:
• square based lamps take square or rectangular shades
• round lamps take round, empire or drum shades.
• bell lamp shades look best on candlestick bases.
Sizing Rules of Thumb for Lampshades from here:
• Shade height should be about 3/4 the height of the base.
• The bottom of the shade should be wider than the widest part of the base.
• Shade width should approximately equal the height from bottom of base to socket.
Of course, we really like oversized shades, but that's just us, what do you guys think Carolyn should do?