Are you going to consider quantity as well? Is that per item, per quantity, or a total for the whole order?
You would just create a shippiing rule defining the charges that you want. If you differentiate based on item size, then you would have to include that size as part of the cart... maybe use the weight field since you probably aren't using it for anything else. Then determine a cut-off weight for medium and large products and charge shipping based on the total or individual column weights depending on how you answer my questions above.
Did this help? Tips are appreciated...
