The object of the components is to make a complex task simple.
The more that is added to the component the more complex it becomes. When this happens the component eventually becoming more complex to operate than the task it was designed to simplify.
With the Fusion e-commerce component if you need different sizes you have to create different items (one for each size and price)
Here is a tutorial that goes over how to manually create a commerce store that interfaces with mals-e (one of the more common merchant processing engines).
http://www.gotfusion.com/tutorials/tut.cfm?itemID=4050
You can create a drop down box for small, med, lg which can send different prices to the merchant processor based on the selection. This would allow you to have one item description with an option where the user can select different sizes with different prices.