OK, I will concede that this plane won't need a .15 . I was thinking about a 2/3 scale model of a .40 size pattern plane that I built that I used a OS .15 FP in. The plane was a Zenith 40, RCM Plan 1212. Wingspan is 37 inches and weight about 32 oz. I had a copy place make a 66% copy of the original blueprint and proceeded to build a plane from that plan.
Nick Ziroli's "Mini Sukoi" had a 34 inch wingspan. I used a Cox .09 RC with throttle and that plane was definitely not underpowered.