I would probably treat it like an equivalent of coke. So if the frozen coke is 16 ounces I would count it as 16 oz of coke. Maybe someone else will have a better idea.
I agree with Sheena, I'm sure it's actually less ('cos of air and ice added to it), but might as well just count it as a regular Coke and get a "mystery bonus deficit" (but don't eat other stuff to fill that haha!)