We seem to have hit the limit with tests such as Tsar Bomba, but that is only because it’s not reasonable or necessary to push it any farther. But I’m wondering what the theoretical limit is. Is there a limit for a single device in which size would make an effective reaction impossible?
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No. Fusion, under the right conditions, is a runaway process, and it’s limited only by its own ability to maintain the heat and temperature necessary for it to continue.
In fact, [for a few minutes at the very beginning of the Universe](https://en.wikipedia.org/wiki/Big_Bang_nucleosynthesis), the *entire Universe* was undergoing fusion at massive rates. During that time, about 25% of the Universe’s regular matter (that is, the part made of protons and neutrons) fused from hydrogen and free neutrons into helium. By a very wide margin, the majority of the fusion that has ever happened in the Universe – despite all its trillions upon trillions of stars fusing for eons – occurred in that 20 minute window in the infant Universe.
Obviously engineering such conditions is far beyond the power of humanity, but it is not (in principle) impossible.
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