tank mpg fuel gallon help

OK, So... I am the new owner of a 2004 F-350 King Ranch Crew. Obviously a 6.0... Here's The issue. Its unknown to me what tank size it actually has.. I've run it to 2 DTE and it only held 23.182 gallons... and then it blew fuel all over my boots -_- Part of my confusion, is the fact that I...