The OEM wheel size for a 1990 Ford F-150 varied depending on the trim level and options selected. There wasn't one single size. To know for sure, you'd need the truck's build sheet or the sticker on the driver's side doorjamb (if still present).
However, common sizes included 15-inch wheels. Sizes like 16-inch were less common but possible, especially on higher trim levels or with optional packages.
