Is there such thing as a slab cake to feed 250 people?, I think the biggest pan I have is for 100.
I have never seen one unless you piece a few large ones togther but the board alone would be to big to transport...I would do 2 or 3 separate cakes...
If you put two 2-layer 12x18's together, creating a 4" tall, 18x24 cake, you'd have cake for approx 200. This is a REALLY heavy cake!
I had a sheet pan that was 14x22 (bought it online). Two 2-layer of these will serve approx 300 and will measure 4" tall, 22x28". When I made this one, I had to take someone with me because I could NOT lift it by myself.
So size and weight is a big consideration.