Most stock puch bing carbs are either 12mm or 14mm. That dimension is the diameter of the 'throat' or venturi of the carburetor.
Moped bing carburetors only have one main jet, and a needle to adjust mixture. They also have an idle speed screw but that's about it. No choke/starter jet, and no idle jet.
15mm bings are aftermarket parts designed to improve performance by allowing you to move more fuel through your engine. They work best when combined with an expansion chamber exhaust.