Consider room style and take the lamp with you when trying on new shades.
Q. I need to replace lampshades for a number of lamps in various rooms. Are there any golden rules I should follow, such as size and height of the lamp in proportion to the shade?
— Margo Rygier
A. It’s always best to bring the lamps along with you on their “outfitting” trip. You are certainly correct that proportion is important when it comes to dressing lamps, as a shade too small or too large can ruin the look of a beautiful lamp. To avoid a shade making the lamp look too top-heavy, make sure the diameter of the widest part of the shade is never larger than the height of the body of the lamp (between socket and base). To even out width proportion, a thick base should be topped of with a shade just slightly wider than the widest or fattest part of the base.
When it comes to shade design and shape, allow your style of the room to dictate to some degree the type of shade you choose. While a large drum shade might look good in a contemporary room, it may seem out of place in a very traditional space. While eclecticism is stylish, mixing modern pieces with traditional has to be more spread out and evident than one lamp in a room. Tall, thin lamps can use either drum shades for a slicker look, or candlestick-shaped shades for a more traditional look.
Also, refer to this great article on Shopping For Lampshades.