The sizes and attachments vary by year of F-150. See sources and related links below to lookup specifics.
It's a 20 in. wiper blade
The 1998 Ford Ranger takes 18" replacement windshield wiper blades on both sides front.
For a 1999 Ford Taurus the driver side wiper blade is a size 24. The wiper blade on the passenger side is a size 20.
The 2005 Ford Explorer takes 20" replacement wiper blades on both sides front and a 16" rear.
19" Front9" Rear
The 1997 Ford F-Series Pickup takes 22" replacement wiper blades on both sides front.
The 1991-2002 Ford Escort takes 20" drivers side and 18" passenger side replacement windshield wiper blades on the front and if applicable, a 14" rear wiper blade.
The 1996 Ford Windstar takes 24" drivers side, 20" passenger side and 16" rear replacement windshield wiper blades.
The 1994-1997 Ford Aspire take 20" driver side and 16" passenger side replacement windshield wiper blades.
The 2005-2007 Ford Five Hundred takes 24" driver side and 20" passenger side replacement windshield wiper blades.
The 1980-1996 Ford F-Series Pickup takes 18" replacement wiper blades on both sides front.
The 2009-2010 Ford F-Series Pickup takes 22" replacement wiper blades on both sides front.