The PeeWee has been used to power 50 inch span powered gliders with weights up to 12 oz. I had a PeeWee in a 30 inch span Taylor cub scale type that weighed 10 oz and it was a HOTROD on it's first and only rudder only flight. The excess power was a large factor in it's demise. I just couldn't control it.

18 to 24 is WAY too small if you are looking at slow scale like flight. For a WW2 fighter type where speed is desireable it would be fine.

If you want to scale up some of the Earl Stahl rubber stuff then you want to go to 36 inch span and don't add much extra structure. That Miss Canada was actually a 36 inch span 130 sq in model. (I have a rubber powered version of it). I would say that using the rubber model style construction you could easily go as high as 160 sq inches and 12 oz. But 12 oz is fairly heavy for only 150 sq inches. You really want to keep it down to less than 10 if you want that slow style of flight. 200 sq inches and 12 oz would be alright but it won't have a fast climb at that point.

Normal numbers support heavier wing loadings but you did seem like you want to keep the free flight look to the flying appearance.