One of our AMS members has a model of the USS Bowfin which measures 12' (144") in length and 5' (60") in the beam, and weighs far in excess of a quarter of a ton (minus ballast).
He transports this monster on a custom built trailer behind his car, and launches it like a full size boat...ie, by reversing the trailer into the lake down a slipway and floating the sub off. To land the thing, he drives the sub into position just behind the trailer and then pulls it back up and onto the trailer by hand.
This beast will be at Brighton Modelworld Show next year, by the way...