The size of shade is based on the size of the lamp. In general, the taller the lamp from base to socket the taller the shade should be. The higher the bulb wattage the larger the top opening of the shade should be, but this is less important with CFL bulbs since they don't get as hot as incandescent bulbs. HERE is a link to more complete info on the subject.

You can also consider a "coolie" style shade, but it may look a bit dated on the lamp.

I think you should buy a new harp, one a bit taller. Nice night table.