You will have to ask, but chances are they will tell you that you have to order according to this schedule: 1 credit = individual pizza, large pizza takes 2 credits, the giant one takes 4 credits. According, to Via Napoli...an individual pizza is meant to serve one person, the large is meant for two people and the giant one is for 3-4 people. Via Napoli is not a Disney-owned restaurant, so they are not very flexible with the dining plan.
We've been there three times now: as a party of two, and then two different times as a party of three. When we were a party of two, we were told we could each order an individual pizza, or we could each use a credit and order a large if we wanted to share. We cound not share an individual pizza. Just a note: we have taken at least 1/2 of the pizza to go every time we've been there...it's a lot of pizza, especially when getting desserts too. They do not have convenient pizza-to-go boxes...it's just entree sized styrofoam boxes so you have to stack the pizza slices.
When we were 3 in the party we were told 2 of us could share a large (and use up 1 credit each) but the 3rd person would have to order an individual pizza or another single entree...we were told 3 could not share a large pizza on the dining plan.
Hope that info helps.