In the eCart support page is the eCart getting started guide. In here look at the section toward the top on adding add to cart buttons to your pages. It covers adding in dynamic options for the add to cart server behavior. If you have a size select list in your add to cart form you can associate it with the item in the add to cart server behavior in the bindings tab. Here is a link to the eCart support page:
If you are interested in learning more about relational databases and how to utilize them from within Dreamweaver you should take a look at our database starter kit. Here is a link to the page:
This and other training materials can be found in the training section of the site.