There is no obvious limit to how big you could scale up this experiment right here. In practice, these circuits are already big enough that you can quite litera

There is no obvious limit to how big you could scale up this experiment right here. In practice, these circuits are already big enough that you can quite literally see them with the naked eye (~mm in length). Nothing really stops you from making one meters in diameter, aside from the obvious impracticality of cooling such a massive structure to the required temperature. Nobody expects this to break any known physics by doing so. In fact, some of these experiments have also been used to estimate lower bounds for nonlinear versions of quantum mechanics (where collapse is a real thing, and the larger an object is, the faster it collapses). But you should not really think of a physical wall in this case. The experiments that have proven the macroscopic tunneling behave according to the same exact math, but nothing is really tunneling through macroscopic walls. The cooper pair electrons are, but that’s a different Nobel prize (Josephson, 1973).