We got the frozen hot chocolate, single size and shared it. It was enough for both of us! We didn't sit inside, we opted for the quick-order window. Line was a little long, and it took a bit longer than I thought to get our frozen hot chocolate.
Anyhow, Serendipity is one of those places that you just have to go to and try. There's a big buzz behind the frozen hot chocolate, but honestly it's not all that great. To me, it reminds me of those chocolate blast drinks from baskins robbins. That why I would say share one with your friends. Don't want you to get your hopes us. It's just chocolate mixed with ice and a little milk.