if you're like me and don't like the factory wheels, Roto, enkei, konig, sparco, and fifteen52 all make wheels for our cars (4x108 lug pattern). you can also go with the stock focus SVT wheels (16" rims, same lug pattern)
you can run 15", 16", or stock 17" rims no problem, a lot of people (including me) choose to go 16 or 15 to get more tire and a nicer feeling ride. but thats all preference.