It is one of 3 things.
1) you haven't uploaded a version of the cart with the Size column
2) you haven't set the value of the size column in the add to cart
3) you have a problem with the code to display the size column
I can't tell which from here, but if you look at your cart object, add to cart buttons, and maybe dump the session which includes the cart onto the page you can probably figure out which of the 3 has the problem. Then from there it should be easier to fix.