Compare it to this sign at Long John Silver's, where the smallest size is 20 oz., and a 32-oz. medium soda, presented as the default size, is nearly 3 times as large as the 1950s king-size double serving (though, as a reader pointed out and I didn't think to mention, we do have to make some allowance for ice in the cup).
The gas station nearest me used to have fountain drink cups that started at 20 oz. I noticed recently they've completely dropped that size; the smallest cup you can now buy is 32 oz. The largest is a whopping 64 oz. I am actually curious how a person gets it home in their car, as I don't see how it would fit in a standard cup holder. Perhaps you buckle it into an empty seat?
I write books. My latest is a YA science fiction novel called Homeland (it's the sequel to Little Brother). More books: Rapture of the Nerds (a novel, with Charlie Stross); With a Little Help (short stories); and The Great Big Beautiful Tomorrow (novella and nonfic). I speak all over the place and I tweet and tumble, too.