I believe the 1976 Fleetwood Brougham was slightly longer (229 -230 inches) but has the same 500 cid. The primary "law" you mention was the CAFE standards of the 1970s (Corporate Average Fuel Economy)that forced fuel efficiency onto the Big 3 and others. It did not outright ban the size or the engine. My 76 Fleetwood gets ~ 10 miles per gallon highway. Simply type 1976 Cadillac into Google and you will find dozens of sites with info and sales on these Caddys. My biggest threat to the vehicle is getting it smogged in California and the brittle plastic on parts of the body - and of course the price of gas.

The only car I can think of that might have been bigger was the Chrysler New yorker. I believe the deville had a 500 C.I.D engine. That's about all I know and I'm not sure about the banning of cars due to size.