16's should fit no issue. There are a lot of people running 15's for their winters.
I typically rely on Tirerack to guide me on wheels. Their winter package for the FiST is a 15x6 which says to me a 16x7 will be fine. The only concern would be offset. Would the buyer let you try them out?Sorry for bringing this discussion back... But I gave incorrect info, the wheels are 15x7... But from what I am hearing, they will still fit. I am really in need of pulling the trigger on this very soon as the weather in Michigan has just taken a nasty turn for the worst... I can barely get the Fiesta moving anymore so I really need to get some winters. Thanks guys!