To get the size into eCart, you will need to create a new column in the eCart object name d "Size" make sure to check the Unique attribute when creating the column.
In the add to cart button, select the Size column and set it to be updatable from a select list. eCart will then add the select list to your page for you.
use that select list in the if statement example from above, and create a hidden form element with the same name for the else statement.
on the bindings panel, expand the eCart section and select the Size column, In the description column of the cart display, place your cursor where you would like the sizer to display, then click the insert button near the bottom right of the display tab.