In the eCart object, create new columns for Size and color and any other options a product may have make sure to set the Unique attribute for these option columns.
Then in the Add to cart button wizard, select each new option column and set them to be updateable from a select list and click OK. This will add a select list to the page, in the property inspector you can populate the list with the appropriate values.
When you create the cart display, on the second page of the Display manager, you should select these option columns to display them in the cart.
