There's no single stock tire size for a 1978 Ford F-100. The size depended on several factors including the trim level, wheelbase, and options selected. To know for sure, you'd need to check the original window sticker, owner's manual, or the tire size stamped on the driver's side doorjamb (if still present).
However, common tire sizes for that year's F-100 were likely in the range of 7.75-15, 8.75-15, or LT235/85R16. These are just possibilities; the actual size could vary.
