First, go to Seller Admin > Cart Shipping and set the difference in domestic vs. int'l rates as the Weight for each box size -- e.g., Small Box Weight = 8.00, and Regular Box Weight = 21.00. Then click Next to save those changes and proceed to the Shipping Rules screen. Create these two Rules:
Domestic shipping:
Shipping Rate Calculator: Price per unit weight;
Shipping Amount: 0.00
Country: United States;
Click Submit to save the Rule.
International shipping:
Shipping Rate Calculator: Price per unit weight;
Shipping Amount: 1.00;
Country: All Countries (i.e. the rest of your Shipping Destination countries);
Click Submit to save the Rule.
Click Back to Admin, and you're done!