Without the dimensions of the foyer and how it joins other rooms, it isn't possible to give you a size. I can say that for a two story foyer, you shouldn't consider less than 36"x36" unless your foyer is less than 7'x7'. The bottom of whatever you buy should be hung at about 7' off the floor. Most entry chandeliers are undersized and hung too high unless a designer is involved.

Best wishes!