I will cover an overview of how you can add in the options and have them affect the price. Please respond with any questions that you have about any part of this and I will be glad to give you some specific detail for those parts.
First, you must have a column in your cart to record the size, and if you want to maintain the price of the size separately from the price of the item you will need another column for the sizePrice.
Once you have the columns in place you will need to ensure that you have a page with the add to cart on it to add an item to the cart. On this page you will need to add a recordset that selects all the sizes for that item. You will add a size select list to the form then bind the size for the label and the id for the value.
Next you will need to add another recordset on the page that will be just like your first size recordset but it will be filtered on the posted size id from the newly added size select list.
Once you have all of this in place you can then add values for the size and sizePrice to your add to cart server behavior. If you want to record the size and price for the size separately from the price of the item you will need to bind the sizePrice column with the price column from your newly filtered size recordset. If you want to just add the price to the items price you can just add in the size price from the recordset along with the already bound price for the item.
Please post back with any questions that you have about any part of this and I will be glad to help you with whatever I can.