Brad
In Practical Junk Rig Hasler & McLeod recommend a maximum size for a single junk sail to be handled by a single person as 600-700 sq. ft. This is largely based on the weight of the sail, and PJR talks mainly about wooden yard, battens and boom. With aluminium, weight may be less, and therefore, presumably, maximum sail area can be greater.
Peter Bailey, when designing a rig for his Spray replica Bertie, wanted to maximise sail area and concentrate as much of it as he could into a single sail. His Hong Kong-style mainsail is 1,000 square feet, with wooden yard and battens. The rest of his required sail area (around 1,600 sq. ft. in total) was provided by two foresails and a sizeable mizzen, making Bertie a Junk Cutter Yawl (!?). Peter reckons the two headsails present more handling difficulties than the main.
Personally, I would not wish to handle a single sail much greater than 700 sq. ft. in area. The building of such a sail would also present difficulties.
Chris