We would like to automatically add a free shirt to the carts of customers who manually add a truck cover to their carts. How can we use Script Editor to elicit the customer’s shirt size so we add the correct free shirt variant to the cart?
I don't think there is a way of adding new line items to shopping cart using Cart Scripts - they are pretty limited.
You could try using something like "Sales Motivator ‑ Free Gifts"
The App store proabably has something that meets your needs or you could develop it yourself.