Imagine you've somehow tranquilized or tied up Superman with a kryptonite rope or something, and the military wants to do a stress test so they can start building a series of anti-Superman bombs. So they drop increasingly large nuclear devices on him, upping the yield until they can noticeably injure Superman and thus determine the upper limit of his invulnerability.
But there's a bigger problem here than mad science or pissing off Superman. It's the limits of our weapons technology. Which leads me to ask.
What's the practical limit for building bombs with today's technology? And I don't mean practical as in efficient or cost-effective. As long as the bomb works and works reliably, that'll do. All it's designed to do is injure a single superhumanly durable target, so it doesn't matter if using it in war on a soft civilian target would be excessive or impractical. Also, saying "antimatter" is forbidden. It has to be doable right now.