As you suggest, size is not necessarily the appropriate quantity to consider. The general idea would be that each player has "valuations" of slices, and that a player chooses based on this valuation. This might result in a sort of sliding scale: a player might choose one piece over another, even though it's smaller, because he prefers the toppings strongly enough.

Are they discrete toppings, or is the path of the pizza cutter allowed to traverse through them?