Measure the dimensions of the furniture that you want to place completely on the rug. You will need a rug slightly longer and wider than the piece to accommodate it.
Choose the location on the foyer floor where your rug will be placed. Try to avoid fixed items like central heat and air registers. Consider whether you want the rug to touch the wall or walls, or if you want floor space around the entire rug.
Measure the length and width of the area of the foyer floor once you have decided where the rug should lay. Write these dimensions down and take them with you to purchase a rug that's the perfect size.