The 1999 Ford Ranger XL came with several tire size options depending on the specific trim level and options selected. There wasn't one single size. To know for sure, you'd need to check the sticker on the driver's side doorjamb or the owner's manual.
However, common tire sizes for a 1999 Ford Ranger XL included P225/75R15 and P235/75R15. There might have been others depending on the wheel and package.
