Most the tire manufacturers recommend a 6-8 inch rim width for that size tire. Seven inche rim width would probably be ideal and six a little on the pinched side....but would be acceptable.
Another consideration, your speedomenter will also be off by 10%. When it shows 60mph you'll actually be going 66mph. The bigger diameter will also affect final gearing and result in slower acceleration and will add a lot of rotating mass (further affecting acceleration as well as braking).
My personel opinion is that they're too big. For a fairly light car like the Focus a narrower snow tire is better. For 15" wheels 195/60-15 is a good snow tire size size. For 16" 205/55-16 works well.