Well, I apear to be a little late, but I can shine a light on the subject if you are still wondering.
You can run 17", but most 17s will interfear with the brakes. About the only ones that I could find were stock take off 17" steal wheels through my ford dealer. Theye were much cheaper then 18" alloy wheels and you won't find 17" alloy wheels that fit. I run the stock 18" alloy wheels durring the summer, 20s really don't have anough sidewall for my liking.
235/80R17 Dean wintercat studded winter tires and I love them! They are over an inch shorter then stock height, and the tread section is 4" narrower then the AT tires I use in the summer, but they work 100x better then any non winter specific tire out there.
2008 F250 Triton 5.4L 3V V8, 5speed Torqshift, Supercab Long Box
Custom Sled deck for winter and IQ checkerplate rack with rails and Betterbuilt Box for summer.
2008 800 Summit X 154 "Improved"
WORK HARD AND RIDE OFTEN