There is a natural tendancy to size boats by length, but when you talk about the size boat that can be easily managed single-hand, I suggest that the limit is more about displacement than length.
I routinely single-hand my 11,000 lb 38 footer and have single-handed boats up to a roughly 24,000 lb, 42 footer, and have raced and cruised on boats into the mid-50 foot range. Based on watching average sized people manning winches and control lines, I would suggest that most healthy people can single-hand a boat up to around 17,000 displacement without too much trouble, but personally I find that 12,000 or so pounds is a lot easier for smaller people to man-handle.
Once you get much over 17,000 lbs, unless you are in very good physical shape, the weight and size of the sails begin to require specialized designs and gear such as low friction hardware and deck layouts, extra purchase on tackles, oversized, or even motorized winches, and perhaps even rigs divided into a larger number of smaller sails.
Its not that a normal person can't handle these larger loads in normal conditions without specialized gear, its just that it begins to become much more difficult and at some point even dangerous in heavy air, especially in rapidly building conditions. Even with greater mechanical advantage, at some point, as these loads increase expedentially, grinding the winches for so long, under such high load, frankly taxes the endurance and eventually drains someone who is not physically fit.
But beyond the mechanical aspects, single-handing gets easier as you develop skills. Often it is a matter of learning to develop an exact timing and sequence to make it easier to perform some manuever or adjustment. Many seemingly difficult tasks become much easier once you learn and become comfortable with where you stand, how you face, where you brace your feet, when to break and when to haul, whether to feather up, or hold course and so on.
And over time, you may make small changes to your boat that help as well. Sometimes, its a matter of leading a critical control line closer to the helm. Sometimes a small change in lead angle can put you in a position to use your body strength more effectively. Sometimes it is something much more major like like having specialized sails made or relocating the traveller into the cockpit so that you can reduce the friction that needs to be overcome when making mainsheet or traveller adjustments.
And it may even require some adjustments to your own body. As I have gotten older, in order to stay in shape, I have added upper body exercises to my routine during the winter.
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Curmudgeon at Large- and rhinestone in the rough, sailing my Farr 11.6 on the Chesapeake Bay and part-time purveyor of marine supplies