The Phalanx (CIWS, Close-In Weapon System) is a 20mm (based on the M61 series Vulcan, same as in the F4, F15, F16, F18, etc).
The Goalkeeper, last I heard, was the follow on to the CIWS. They were talking about mounting the GAU-8 Avenger (the 30mm from the A10 Thunderbolt II/Warthog) with the same radar seeker/targeting setup. Or two GAU-8 with one rader.
Oh baby.
For dragons, though, I agree with PJ11B3VF7...the ZSU-23-4 would be just about right. Harppon gun on the back of a truck...yeah, right.
Alex