Fixture Sizing: What is the best size fixture for my space?
Over an Island or Bar:
Fixtures should be no lower than 30" off of the surface but not so high that they do not effectively light the area. General rule for a bar style fixture is: 12" narrower than the length and width of the island. For example, if the island is 36" x 60", the fixture should not exceed 24" wide by 48" long. This way the fixture does not feel as if it is invading your space. It also saves a lot of bumped heads. For Islands with multiple pendants, the same rule applies as far as "not to exceed" length and width. The question with pendants often becomes "How Many? & How Far Apart?" Well that depends on the pendant and the electrical. My opinion is that three always looks better than 2 and that there should be the width of the pendant between each pendant. BUT... with so many styles and sizes of pendants out there it is like putting the cart before the horse.
Living or Bedroom:
In general, take the width and the length of the room in feet, add those two lengths together and convert to inches. That is the basic size fixture you should purchase for the space. For example, a living room that is 15" x 12" = 27". So your ceiling fixture should be approximately 27" in width. Of course, rules are meant to be broken! One rule that should not be broken is fixture height. Ceiling fixtures, unless over a table, should not hang below 7" from the floor. This is so you do not hit the fixture when passing under or around it.
Over a Dining Table:
I believe in "Go Big or Go Home" in dining rooms. Is is the focal point of your room so don't undersize or under-WOW! So I suggest you start with "How Big can I go?". Take the width of the dining table and subtract 10" to get the maximum width for a fixture (Go too wide and you will have bumped heads). To determine the maximum height of the fixture, we must look at the ceiling height. A general rule for dining fixtures is to hang the fixture so the bottom finial is at 5" off the floor. If we have a 10" ceiling in our dining room, we would subtract 5" for the hanging height. That would leave 5" to work with. I like to see at least 12" or so of chain on a fixture. Otherwise it looks like it is too close to the ceiling. After all the subtractions, you will end up with 48" to work with.
Now... All that being said, dining rooms are also a place to try something different. Maybe 2 fixtures instead of one, a drum shade fixture or even an antique lantern style fixture. Whatever you decide, think about safety (no bumped heads), make sure it will light your dining room and most of all make sure it will WOW your family and friends.