In determining the right size chandelier for your room, add the dimensions of the room together. The sum equals the diameter of the lighting fixture you will want. So, if a room is 10' x 14' then the diameter of the fixture should be at about 24 inches.
Here are few rules of thumb:
A room smaller than 10' x 10', use a 17" 10 20" wide fixture.
A room that is 12' x 12', use a 22" to 27" wide fixture
A room that is 14' x 14', use a 24" to 32" wide fixture
Hang the chandelier approximately 7' off the floor. Same applies for the foyer and if it is an open, two story foyer then the fixture should not hang below the second floor. If you want to hang it a bit higher then consider adding a few inches to the diameter so it doesn't appear small.
Dining room chandeliers should be between 1/2 to 2/3 of the width or diameter of the table. Hanging the chandelier should be between 30" and 34" off the top of the table assuming you have an 8' ceiling. For each foot of additional ceiling height , add 3" to the suspension height. If you want to hang two chandeliers then select chandeliers that are 1/3 to 1/2 the width of the table.
For Kitchen islands, the fixture should hang with a 30"- 34" clearance between the top of the counter and the bottom of the fixture; remember that counter top heights can differ. The size and number of fixtures can vary here, just set them at even widths apart based on the maximum diameter or width of the fixture. Be sure to leave at least 6 inches of counter space on each end once it is hanging.
For wall sconces, the bottom of your sconce should be a minimum of 60" from the floor and should be at a level where you are not looking directly at the interior bulb. If you are placing sconces at your bedside over overhead then the best way is to literally sit in your bed and determine what the best height is for you. There is no guideline here as overall mattress heights vary quite a bit.