* Check the owner's manual: This is the most reliable source.
* Look at the fuel tank itself: The capacity might be stamped on the tank. However, this requires access to the tank, which might be difficult.
* Check the vehicle's sticker: Some information stickers inside the driver's side doorjamb might list the fuel tank capacity.
Without knowing the specific van's details, it's impossible to give a definitive answer. Expect it to be somewhere in the range of 20-30 gallons, but it could be more or less.