In the cart object you should have a column for the Size.
Make sure that column is included in the cart display as well.
In the Add to cart button behavior, set the size column to be updatable by a hidden element.
the add a javascript onchange event to the price select list to populate the size hidden element:
from you example, i know the form name (InsoleCart_1_ATC_0) and the select list name (InsoleCart_1_Price_Add), so the on change should be:
onChange="document.InsoleCart_1_ATC_0.<hiddenFieldName>.value = document.InsoleCart_1_ATC_0.InsoleCart_1_Price_Add[document.InsoleCart_1_ATC_0.InsoleCart_1_Price_Add.selectedIndex].text"
where <hiddenFieldName> is the name of the hidden form element.