A foyer chandelier is a great way to light up an entryway, but many people don't know the right height for hanging one.
In most rooms chandeliers should be hung about 7' above the floor, but this isn't always the case in the foyer. While it should certainly never be lower than 7', it can sometimes be higher. For instance, if the foyer is two stories the fixture should not hang below the second floor. If the foyer is just one story but the ceilings are high you should follow the rule of thumb that there should be at least 7'6" from the bottom of the chandelier to the floor. Remember also that a foyer chandelier should always be hung in the center of the room.
Since chandeliers will look smaller the higher they are hung, you might want to add a couple of inches to the diameter of the fixture if it's hanging quite high. Normally you would base the width of the foyer chandelier on the overall dimensions of the room. Add the width and the length of the room in feet and then convert the answer to inches. The answer in inches will be a good width for the chandelier.
It also makes sense that the higher the ceiling the taller the foyer chandelier should be (it will look more balanced). A common rule of thumb is to allow 2&1/2" to 3" for each additional foot of ceiling height. So given that a room with an 8' ceiling should have a fixture about 20" to 24" high, then a 10' ceiling should have a fixture about 25" to 30".
Remember that these are only guidelines. Always assess how the piece looks in your space and feel free to play with the height a bit. Depending on the chandelier size, you might want to hang it a bit higher or a bit lower. Ultimately you will be the one to decide what looks best in your home.