In general, the taller the lamp, the larger the shade.
Most table lamps take a shade with a bottom diameter (B) of 16" or less.
Floor lamps take a shade with a bottom diameter (B) of 16" or 18" or larger.
Measure the lamp's height from the bottom of the base to just below the socket(s). The basic rule of thumb is that the shade you choose should have a bottom diameter (B) that's approximately equal to this measurement.
Shade risers can be purchased at your local hardware store to tweak the shade a bit higher if necessary.
The lamp's fitter is the most important measurement and how you begin searching for the right shade.
Measure the the diameter of the lamp's fitter(s). This is the cup, ring, or other holder into which the glass shade is inserted.
There is no other rule of measurement for other dimensions of the shade(s), except to say that the larger the lamp, the larger or longer the shade should be.
The primary consideration for choosing a desk lamp and its shade is finding one that will accommodate a high wattage bulb and will focus the light downwards. You will need a total wattage allowance of 75-150 watts.
For fabric shades however, the closer the shade is to the bulb, the lower the wattage bulb you will need to use. Approximately 4-5" around each bulb is good breathing room for a fabric shade for use with a high-wattage bulb.
To measure for the right amount of light on the desk, a distance of 15" from the desk top to the shade's bottom is about right.
The size of a dining room hanging fixture depends on both the size of the room and the size of the table. The perfect hanging fixture will be a happy medium between these two measurements. For determining the size of a hanging fixture based on your table size, it should be approximately 12" smaller in width than the width of the dining table.
Other hanging fixture should be smaller in scale when used in hallways and other rooms for general lighting.
Candelabra shades reduce glare and help to direct light down where needed on both hanging lamps and wall sconces.
However, a 25 watt bulb is the highest wattage allowed for safety with these small shades.
Multiply the total number of bulbs on the fixture by 25 to get the total amount of light your lamp will be giving off.
Adding fabric candelabra shades adds about 2" to 4" to the total width of the chandelier.
