I agree with Mr Cox.
The power loading chart really is only relevant when working with watts or horsepower, not engine size. Some 1/2A engines are 3 times more powerful than others.
How each engine is propped further complicates the matter, because that becomes the final developed HP.
A 26 inch span model with 130 sq inches will normally come out weighing somewhere between 7 and 12 ozs if outfitted with 1/2A sized equipment [2 servos] and depending on construction techniques. The lighter, the better...anything over 12 ozs [at this size] will not very well. Most guys here who have written about scratch built planes in this size range have had no problem building them at about 9-11 ozs and they fly tremendously.