for a 1994 (guessing since that is your profile) the 3.0L takes ford p/n E1FZ-6379-A (fits 1998-2008 both straight gas and flex fuel).... I know that doesn't answer the size, but it is the info I can find without tearing a truck down and measuring by hand... could ask Ford service dept or lookup on parts geek, rock auto, etc what the specification is.
If you need a different engine, might tell us which... if you need for a specific trans / clutch combo might tell us which of the manuals it is ESPECIALLY if it isn't one of the Ranger ones and you put in a F-series or some other swap that might make it different.