Great image that!
But I don't know what is the largest possible either. With the two 26ft Bolger Martha Janes I've had experience with they responded pretty much to the same thinking with the GIS above despite balance lug mains around the 190sq ft mark.
Actually the second one drew many complaints from the owner as it was as slow as a wet week - until he asked me out for a sail and I found the downhaul was quite inadequate - so we added a cascade purchase to double the mechanical advantage and told him to use it "brutally" as the wind got up and he's been happy with the sailing performance ever since.
The only disadvantage was the tricked up one for ocean cruising (more ballast, no cockpit) had the problems with chafe that I mentioned.
But as far as a 125sq ft lugsail it isn't that bigger jump from what we've been looking at above.
The only caution is that the yard in particular and the boom to a degree should be flexible enough to bend in the gusts so the sail depowers automatically and powers back up as the gust fades.
One GIS builder used very heavy gauge aluminium tube (think it was 2" with a wall just under 1/4") and found that his GIS just wanted to fall over in gusts even with a load aboard. I think I worked out at the time that the alloy spars were around 10 times stiffer than the original tapered wooden spars specified.
The tapered timber GIS spars bend around 1 1/4" just under the effect of medium wind downhaul tension (105sq ft). With my sailing canoe where everything is pared down to keep weight down there is probably close to 2" bend in the same conditions (68sq ft in the main).
With smaller balance lug sails like the mizzen on my sailing canoe there is virtually no spar bend in any condition. To be strong enough for the stability of the boat they end up very stiff compared to the load that the smaller sail can put on them - but that's often the case with mizzens anyhow on boats of all sizes.
Hope that suggests some useful things to observe.
Best wishes
Michael




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