A: Shades typically add 2-4 inches to the total width of the chandelier. To find the right size for your room, including the shade, add the dimensions of the room together. So it a room is 14x20, you'll need a chandelier of 34 inches in diameter. Don't forget to account for the shade in the overall measurement.
A: Chandelier shades reduce glare and help direct light down where needed. If you need more light, consider the level of opacity. If it's completely opaque you won't get very much ambient light. A clear or beaded shade will provide much more. But the shade you choose is largely a personal choice. How it looks with your decor and your style is the biggest factor.