The Kennedy class boats are 265 feet from end to end.
The Barberi class boats are 310 feet from end to end.
When I lived in NYC I always thought the SI Ferries were the biggest things around....then I got out to the west coast and visited Seattle.
Biggest ones in Washington State Ferries' fleet are 460 feet length, about 75-80 feet width. Heck, they have EIGHT car lanes on the deck, with two two-lane ramps over the outer lanes....218 cars total capacity on each one of the ships!! (These are the Tacoma, Puyallup and Wenatchee).
Previously, the Spokane and Walla Walla were the largest US ferries at 440 feet, 206 car capacity (very similar deck/car ramp layout).
I think the smallest boat in service up at WSF holds 45 cars....