That is just one way to do it if you have the size data in a separate table. You can do it much more simply directly in the code by checking on the size the user selected, then adding 1.50 to the price.
The code would need to go in your add to cart server behavior that is on the page and the specific code will depend on th name and value of your size select list, here is an example of what it might look like:
$ATC_itemPrice = floatval("2");// column binding for the price from the add to cart
if(isset($_POST['size']) && $_POST['size'] == 'xxl') $ATC_itemPrice += floatval("1.50");
If you post back with your add to cart page that has the size select list on it I could show you a more specific example.