Instead of trying to match the U.S. market with a foreign market of the same size, just require the passenger count to be the same, e.g. AC can fly 100,000 people a year if U.S. airlines can fly 100,000 people a year on Canadian domestic routes.

We would also need to require the foreign airline to not be subsidized by the government. Otherwise the result is U.S. airlines are forced out of business, or the federal government has to do the same and susidize American carriers.