The details on the non-individual pizzas have not yet been released. I would assume (like above) that the individual pizzas will be 1 credit on the system. Following the trend of other multi-serving entrees and desserts, I would guess that the 20in. would be split between 3-4 people while the half meter between 4-6, and thus requireing that many credits (and people in the party) to use. That is if the larger pizzas are on the plan at all.
Again, that's only a guess based on other instances of multi-serving courses (mostly desserts and apps right now) and I could be way wrong. I would feel safe in saying that you wouldn't be able to get one of the larger pizzas with 1 credit though.