As the width goes, take the Width of the foyer + the Lenght = Width of the chandelier. So if you 11' wide + Let's say 14' long the width should be 25" W and as the height goes, you can be in the middle of that window or in the middle between the top of the door and bottom of the window.

Your home leans traditional. That contemporary chandelier is not a good fit.

I think the chandelier you like is too contemporary for the space. It does not work well with the stair railing or window beyond. Try looking a chandeliers that are more "transitional". I do like that you are thinking out of the box and trying to get away from a traditional shape. Hudson Valley Lighting has some really nice options. I picture something more Lantern shaped.