Do you have the values associated with the sizes in your db? If so you could create a new recordset to select the sizes from this table. This recordset will be used to populate a select list in your add to cart form for the size. For the value use the id of the size nad for the label use the size name.
Once this is in place you will want to add another size recordset that is just like the first one but it will be filtered on the posted value of the size select list, it will post the value that the user selected.
You can then use the bindings of this second size recordset in your add to cart server behavior to add the amount to your size amount column, and the name to your name amount column. You could also just add the price to the items price rather than having a separate column for the sizes price.
This is an overview and you will need to be able to craft the recordsets and setup the select list and add to cart server behavior. If you have any questions about this please post back with specific questions starting with the first steps. It is a good idea to work on this one part at a time rather than trying to understand and implement it all at once.