I have not come across "tight" guidelines for ruder size for sailboats . . depends to a great degree, I suggest, on the underwater configuration of the boat, the expected speeds, and the type of rudder. In the old days, you could find a Lloyds formula for the strength of the stock, which would also give you a pretty good idea of the strength requirements for the rest of the rudder.
I would not trust any boatyard welder or laminator to make those judgements by eye. I would want a naval architect involved.
Rudder loads can get pretty high . . . which is what makes it so difficult to concoct a jury rudder that will last longer than a few minutes.
In the case of Viewfinder, do we know what happened to the third rudder? Maybe it did break off at the hull . . . otherwise the quadrant, etc., would have prevented it from falling completely out, no?