The Greeks discovered the secret,they called it "The Golden Mean or ratio". It says that you should think in percentages for color, mass, and even texture.
You should have about 70% of any one thing; 25% of a related thing, and just 5% of a contrasting thing. Two quantities are said to be in the Golden ratio, if "the whole is to the larger as the larger is to the smaller". Equivalently, they are in the golden ratio if the ratio of the larger one to the smaller one equals the ratio of the smaller one to their difference.
A great architectural example is the parthaenon basically the golden rectangle like a credit card phi on one side 1 on the other... a geometric example would be the pythagorean theorum, the square of the hypotentuis is equal to the sqaure of the oposite two sides (if memeory serves me? I know I'm a bad speller)
Anyway, you can work with those numbers as you see fit, for example take the total area of the walls and divide. Walls=70%
Openings=25% Trims=5% Or you can take the height of the walls, 8' divide that by 5% you'll get 4.8 so a 4 1/2 crown?
You may want to work with the colors of the room? But the golden mean is a classical standard of design & balance.
Basically here is the rules for a ceiling eight feet in height, make sure that the crown molding is six inches or less in width and the base molding is five inches or less. Set chair railings 36 inches above the floor. If your ceilings are lower than average, the railing should be lower to make the wall seem taller, but you shouldn't go any lower that 30 inches.
When it comes to installing the crown molding I have a tutorial on my site that is fairly popular just put "installing crown molding" into a search engine.
Alter Eagle Construction & Design