If it's the original engine use the VIN number and go to your favorite parts supplier and they can tell you. If it's been changed you have to get the casting number and do some research. Or go to your garage and ask your mechanic and he will be able to tell.
very carefully
The 2006 Ford Focus has a 2.0 L base engine size.
The 2005 Ford Focus has a 2.0 L base engine size.
The 2001 Ford Explorer has a 4.0 L base engine size.
The 2013 Ford Mustang has a 3.7 L base engine size.
The 2012 Ford Taurus has a 3.5 L base engine size.
The 2007 Ford Taurus has a 3.0 L base engine size.
The 2014 Ford Expedition has a 5.4 L base engine size.
The 2009 Ford Flex has a 3.5 L base engine size.
The 2007 Ford Fusion has a 2.3 L base engine size.
The 2006 Ford Explorer has a 4.0 L base engine size.
The 2011 Ford Fiesta has a 1.6 L base engine size.