We'd count the number of Mexicans here illegally. Take a factor related to "square foot per Mexican" based on how big Mexico is and how many Mexicans there are and then multiply that number against the number of Mexicans inside the US.
Anybody living on that territory now would be EVACUATED SOUTH and the fence would be built to enclose and secure that land from the Gulf of California to the Gulf of Mexico.
Mexico can reduce the amount of space by taking back their people.
Great idea, I’m for that. I have hunted arrowheads on both sides of that river. The best ones I found on the Mexican side.