Ever seen those "WIDE LOAD" trucks on the freeway? Probably about that wide...
I think they might be able to give you trouble if your tires are wider than your fenders, though. Sometimes those guys with lifted trucks get in trouble for not having mudflaps. I don't know if that's a height or width issue though.
All I know is most states require you to have fenders that extend over the width of your tires. So, if you get wider tires, you need wider fenders as well.
I always wondered... you know those low rides with the little mud flaps that stick out at a 45 degree angle and look so weird, but they have tires sticking way out? Maybe that is why they do it - to have "fenders" that are as far out as the tires.