I see gaint scale airplane adds recomenting 2.7 to 3.2 for aircraft weight of 15 pounds. I have been flying 18 pound airplanes with successfully with 1.8 engines. A 3.2 to 3.6 should fly 22 to 26 pound planes withour difficulty. Are they rating engine size for only 3D flight or am I totally wrong?

Depends on what kind of giant scale you are wanting to fly.

If it is scale aerobatics, you will not have enough power to fly an upline with snaps or any other maneuver with the power you are talking about.

I have a 26 lb Edge 540T with a 100 cc 6.2 ci engine and I wouldn't want anything smaller on it.

If you want a war bird or any other type of scale plane and want it to fly scale, you would be fine with one of the engines you mentioned.

I have a Joe Bridi killer Bee that wheighs 16 pounds and have unlimited verticle and more power than I want with a super tiger 3000. Maybe with the type of flying that I do there is no need for more power. I'm just an old guy and just do loops cuban eights ect. even with an extra or edge.