I think this might be the wrong question to ask. Or at least, an insufficient question to ask. Perhaps at least as important as the tank size is the degree of beginner and their personality.
The "degree of beginner" aspect will sharply influence the need for the stability of a larger tank. When I set up my first reef, it was a 15 gallon high in 1990. But I had decades of experience keeping all sorts of freshwater fish, including some quite delicate species. I was already acquainted with the nitrogen cycle, the need for oxygenation (flow), the need for specific gravity stability, temperature, etc... And I have an analytical nature. That allowed that 15 gallon tank to succeed beyond all expectations, despite its small size and the rather crappy equipment choices back then (the "Skilter" was the only choice for a tank like that!).
However, I would very much not recommend such a small tank to someone that's never kept fish before, much less saltwater. There's a great deal to learn, and as other posters have pointed out, volume=stability in this game.
As to the personality question, I think that very much influences the ideal tank volume choice. Someone that is fastidious, highly interested in details and consumes the available information like a dog with a bowl of chopped steak in front if it will likely do quite well with a nano tank. Someone that has only a little interest in the details and just "wants instructions" is likely to do a lot better with a 75b - 90g tank, where their mistakes will have considerably less of an impact. Another aspect of the personality question is their penchant for frugality - note that's not the same thing as their disposable income. Someone that feels the pain acutely of purchasing a $300 pump or an $800 light will likely be very reluctant to spend the money that it takes for a lot of the test equipment and ancillary equipment that is critical to successful reefkeeping - one of the biggest examples of this would be a RODI system. Equally problematic might be someone that has quite the opposite mentality and simply pulls out the credit card without really researching why they want or need something, and then is really frustrated when that item is neither easy to operate, nor fixes the perceived problem.