Add Variations to a Product

Hi I am looking to ad variations or sizes to products in a dropdown or radio button format in marketpress. the last wordpress ecommerce type plugin I used offered a dropdown on a product for various versions. A good example would be

Product: A really great Tshirt
Size: (dropdown)
Sm
Med
Large
XL

On selection of a given size, price would adjust accordingly. If this option isn't there, I would need to add a product for every variation (or size)!