Yes, boat length probably does not come into it as much as weight of the sail as in what has to be pulled up the mast, number of able crew to assist with sail handling, and I also suspect sail configuration. Some of the photos of genuine Chinese junks of the last century show some of them having massive sails but I expect there were a lot of crew to handle the sail and I understand that hoisting the sail took a very long time.
Footprints at 10 meters has a 53 square meter single sail of the fan shaped configuration. I can handle this OK myself but I find that hoisting the sail to be the equivalent of a good gym workout. The fan shaped sail on Tystie was 57 square meters for a little more boat length than Footprints and I very much admire David Tyler for the way he was able to manage that sail single handed on ocean crossings. The German boat Peregrine I think has a sail of about 80 square meters and that sail is of a taller aspect ratio with I think a solid timber mast as the mast would need to be very strong for this much sail area. But in practical terms I suspect that 11 meters or so would be the largest hull size for a single sail rig unless of course one was to build the rig of high tech materials, (i.e. Carbon), to allow for higher strength lower weight but then it would be a very expensive rig which is getting away from what the junk rig is all about.