RE: .15ci / 2.5cc size C/L Questions
If you're talking about sport/stunt models, then quite a lot depends on what sort of .15 you intend using. Popular 230-250 sq.in designs like the Jr Flitestreak and Akromaster work best with a light, unmuffled engine such as a Fox .15 or OS Max .15 (or a good 09, as Jim alludes to). Throw on a heavier motor and/or a stock muffler and they won't fly nearly as well - you need tail weight to get them to balance properly, and then the wing loading starts to become excessive. You'd still have something that's easy and fun to fly, but not as good as it could be. As an example, the current Enya SS15S or 15-V are very nice engines - but with stock muffler, weigh a touch over 200g, which is getting on for double the weight of the aforementioned Fox or OS without muffler. I can't get away without using a muffler, and prefer the way my Jr Flitestreak flies with a good 09 to how it is with the much more powerful SS15. I ended up designing a flapped 340 sq.in., 26 oz model for the SS15. No tailweight required and it flies great, even though it's a bit bigger than a traditional .15-size model.
Cheers
Steve