* The sticker on the driver's side doorjamb: This sticker lists the recommended tire size and inflation pressure for your specific van.
* Your owner's manual: This manual will also specify the correct tire size.
* The sidewall of your existing tires: The size is printed on the sidewall of each tire (e.g., P235/75R15).
Without access to your van's documentation or tires, I can only say that common tire sizes for a 1998 Ford E-150 were in the range of P235/75R15 or similar, but variations exist. You *must* check your vehicle's documentation to be certain.