Hi @ll !
I have been asking myself exactly this question for some time (and now I finally have the chance to start testing it in the real world). Last year I bought about 5.8 hectares (roughly 14 acres) in the Spanish Pyrenees. It is mostly permanent mountain pasture, with some woodland, a natural spring and an old stone building that I am slowly beginning to bring back to life. I am an agronomist and also an electronics guy (so perhaps my way of looking at all this is a slightly strange mixture of old and new), and I am trying to understand what scale of land can give you freedom without turning that same land into an endless source of work.
My goal is not complete self-sufficiency. I am not sure that producing absolutely everything yourself is necessarily freedom (sometimes it can simply become another full-time job). What I am trying to create is a place that will need few external inputs, can produce a good part of what I need, gives me useful work rather than endless work, and still leaves enough time to actually enjoy being there. At the moment I am still very much at the beginning, so many things are ideas, some are starting to be tested, and others will probably change completely once I understand the land better.
The land itself is already setting many of the rules. I have around 3 hectares of good pasture, some woodland, plenty of water from the spring, and a mountain climate with cold winters and dry periods in summer. So the idea is to work with what is already there (instead of forcing the land to become something it does not want to be). I am considering rotational grazing with sheep and/or goats, bees, a relatively small vegetable garden and orchard, improving the pastures, using the woodland carefully, and managing water so that gravity can do as much of the work as possible. None of this is fully established yet, and that is important for me because I do not want to design everything from a desk and then impose it on the place.
Energy will probably be mostly solar, and here is perhaps where I differ a little from the romantic idea of going back to a completely low-tech life. I love simple systems (very much), but I also like technology when it removes stupid repetitive work. So I am planning to experiment with things like sensors, electric fencing, GPS livestock collars, solar pumps and eventually small autonomous machines for mowing or pasture management. Not technology instead of nature, but technology to allow nature to do most of the work.
For example, my idea is that if a small solar-powered machine could spend several days slowly cutting pasture while I am repairing a stone wall, looking after animals in the future or simply walking in the mountains (which is also part of the good life), that would seem much more interesting to me than spending those same days sitting on a tractor. The same applies to water: I am thinking about systems where gravity, a tank, a few valves and a small controller could irrigate only where and when it is needed (without me spending the day opening and closing things). I do not see that kind of technology as making the system more artificial; I see it as a possible way of interfering less.
I am also beginning the process of restoring an old stone building rather than building a new house and, eventually, I would like the place to be useful to more people than just myself (a place to share practical knowledge, experiments, tools and mistakes too). I would like to show that rural life can still be attractive and possible without pretending that we have to live exactly as people did 200 years ago. For me, the interesting part is precisely that combination: old stone, pasture, water, animals, woodland and very simple technology used only where it genuinely helps.
So, after buying 5.8 hectares, I certainly do not have an answer yet. But I am beginning to suspect that the most important question is not simply how much land a good life requires, but how much land one person can care for well without the land eventually becoming his master. For me, 5.8 hectares currently feels like a very interesting scale to experiment with (perhaps not the perfect one, but a very interesting one): big enough to imagine pasture, some animals, woodland, water, food production and some genuinely wild space (which I also want to remain wild), but still small enough that I hope to understand what is happening almost everywhere on it.
At least, that is the theory. Ask me again in ten years (if the goats, the bureaucracy and the mountain have not defeated me first) and I may give you a completely different answer. That is part of the experiment.