I'm an intermediate flyer and find this question to be interesting to me. So many will say, put the biggest engine you can into the plane, and maybe even the next size bigger. I learned how to fly RC with a Sig Kadet Senior that I built and I have a Thunder Tiger GP42 in it. That's a 78" high wing trainer, and now that I look at it, the GP42 looks small in it, but it flew it fine, I had a blast with it, could fly really slow and pretty much taught myself to fly with it. I still have it, and it still looks good as well in flyable condition. My next plane was an Avistar, another Thunder Tiger GP42 was used, I flew the heck out of that plane. Many put a 46 into Avistars, but I never felt I needed a "rocket". It flew fine on a GP42. It was also more economical to run compared to my TT PRO 46. The engines made today are much better than what was produced 15 years ago. A strong 46 today is like what a 60 was years ago. So I'm not convinced if the kit box says 40-55, that the best solution is the 55. It's probably personal prefference that is the main difference. So many things have to be taken into consideration. The plane itself, how you like to fly (a rocket with wings or fly on the wings), what you want to do with the plane, altitude of your field etc. I would say, it's probably not a good idea to choose the smallest engine size. I can't imagine my Kadet Senior with a 29 on it. Probably would fly, but I fly off of a grass field and that's another thing to consider. Definitely be within engine size the model suggests, don't be on the low end of that scale. I'd like to hear others thoughts.