To add options to a static, you first need to add a column to the cart object for the option.
In the eCart object editor, go to the Columns section and add a new size column. Set it to be unique.
Now in the eCart Add to Cart button wizard, select the new size column and select the option to be editable from a select list.
This will add a select list to your page. you can then populate the select list with the available sizes.