Well for my 1992 E-150 the factory put in a 35 gallon tank so I am guessing you have the same year van as me so to answer your question it will be 35 gallons.
it has a 35 gallon tank
35 gallon
My 1992 E150 has a 35 gallon tank so I'm guessing your E150 is the same even though it is only one year newer than mine.
19 Gallons
16.0 Gallons
20.86 Gallons
12.8 Gallons
Mine hold 17 gallons.
13.2 gallons ( U.S. ) in a 2002 Ford Focus
12.4 U.S. gallons on a ( 2012 ) Ford Focus
14 - 15 gallons
19.3 Gallons