If a whole room will be lighted by the fixture, you need to first measure the room's width and length and then add the numbers together. You then have to convert the result from ft. To inches so at to know the proper diameter of the light source to purchase.
However, if the fixture will be centered over a furniture piece, you'll need to measure the width and length of that particular piece instead of the whole room.
When it comes to aesthetics, you are the one that'll choose what you think is beautiful. However, you need to ensure it matches the existing style of that particular room.