You could probably get away with one foot. Another option is to have the panel company leave pockets in the top of the roof sip, where you can insert 2x4s that then cantilever out to be the rafter tails that support the overhang. I've done this, and it's a PITA compared to just cantilevering the SIPS themselves out to form the overhang. I think you can go 2 or 3 feet with the latter method, as the SIPS are so stiff. CB.