To dump the session add this code to any page... maybe create a new debug.php page:
After adding an item with a size to the page, go to that page and see if the cart stored in the session has the size. If it doesn't then you know the issue is with the add to cart or cart object itself. If you see the size in there, then the issue is with the cart display.
I just tried your page and the size values are showing up for me in the cart.