15" is the recommended size although, a little longer wouldn't hurt.
Wiper blade sizes will vary depending on the year of the Ford F150. See sources and related links below to look it up.
The 1985 Ford F150 wiper blade sizes are 18" wiper blades on both sides front. See sources and related links below for more information. Since there are over 22 different wiper arm attachments, do not buy wipers just by length only - be sure to lookup exact wiper blade part numbers by vehicle make, model and year for the brand wiper you decide to purchase. This ensures correct length AND correct attachment.
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 1993 Ford F-150 has the most common hook-end wiper arms. See sources and related links below for installation videos. Locate and press the tab or button where the wiper blade is inserted into the hook of the wiper arm and pull wiper blade out of hook. You may have to rotate and flex the blade to make enough clearance for the hook to pass through.
I do not see a rear wiper blade listed for a 2008 Ford Focus. If there is one on there, it must be a special Ford-only wiper (which is rare for Ford - they normally go with universal wipers). I do see that 2000-2007 Ford Focus has a rear wiper listed for both the hatchback and wagon. So, possibly you have the year wrong? If so, the wiper blade videos for hook-end wiper arms will show you how to change the rear (as well as front) wiper blades.
18"
It's a 20 in. wiper blade
The 1991 Ford Aerostar has the most common hook-end wiper arms. See sources and related links below for installation videos. Locate and press the tab or button where the wiper blade is inserted into the hook of the wiper arm and pull wiper blade out of hook. You may have to rotate and flex the blade to make enough clearance for the hook to pass through.
See sources and related links below for wiper blade installation videos and for wiper blade information - the rear wiper is relatively hard to find in stores.
top of the fire wall under the hood your wiper arms will connect
can a ford f150 seat fit in my 1978 ford f100 custom truck
The 2009 Ford Escape rear wiper blade size is a special 12" rear wiper blade that is very simple to replace once you find a replacement. Unfortunately, most stores do not stock this special rear wiper blade so please see sources and related links below for more information and installation videos. Once you get the wiper blade, there will also be step-by-step instructions on the packaging.