What's your plan for a solution?
You must FIRST decide on a "serving size". Is it a "slice"? Is it "square inches"? What?
Once you have decided, then you can solve the problem.
Yet another Area solution.
Perhaps this option... One serving = Number of Slices
8" Pizza can be cut into 6 slices. 6 slices / (2 persons) = 3 slices / person
12" Pizza can be cut into 8 slices. 8 slices / (3 slices / person) = 8/3 persons (or 2 persons with some leftovers or 3 persons with one a little dissatisfied)
Perhaps this option... One Serving = Disk or Washer of equal net diameter.
8" / (2 persons) = 4" per person
12" / (4" / person) = 3 persons
Who ever said anyone eats pizza the same when it comes in different sizes?