Simply add the dimensions together. For Example, if foyer is 12’ X 16’, add the 12 plus 16 – to get 28. The Proper size chandelier would then be 28” in diameter. This works for any room. Make sure the bottom of the fixture does not hang more than 7 feet from the floor. If windows are present, center the fixture in the window so that it can be seen from the outside or choose a height that will give you the best look.
Another concept is to divide the width of the foyer by 3. For example, if the foyer is 12 feet wide, a 36” chandelier would center nicely with 36” on both sides.