If you sell two sizes of something (I don't care if we're talking pizza, soda or fat hookers), you sell a SMALL size, and a LARGE size. You do not, by definition, sell a MEDIUM size and a LARGE size. Websters defines that ever confusing term "medium" as:
1 a : something in a middle position b : a middle condition or degree
If you in fact sell two sizes of, oh let's say, soda. And someone orders a "small", you are *not* allowed to reply "We don't sell small, we only sell medium and large. Its ('it' what?) the same price as a small".