Since most people want the medium size, call it a medium. Don't get cutesy with your offerings. When you offer medium, large or jumbo you only confuse your customers. Follow the standard naming conventions of your industry.
Making your own names not only frustrates people, it makes you look kind of snobby. If they were smarter, they would know how to ask for the right product. So unless you are trying to be an elite company that narrows its customers to a select few, just give me a medium.
Do you have special naming conventions that only a select few understand? How could you make them more accessible to all of your customers and new prospects?