I tend to suggest beginners with gas engines get their feet with a 20cc. The cost difference between 10cc and 20cc is not that much, the 20cc will have a needle bearing connecting rod so 32:1 can be used, the 10cc will require 16:1 which means more mess. The 20cc will have a stock Walbro carb as opposed to something unique to the engine. Walbro replacement carbs if needed are cheap. The larger airplane will also be much easier to work on. It would be a bit of a challenge to fit all the radio gear, fuel tank, ignition module and seperate ignition battery into a .40 size airplane. While on the subject of ignition batteries, while there is nothing wrong with running a seperate battery, it just is not nessesary. The current ignition modules are no where near as power hungry as they used to be. I would venture that the majority of gasoline power airplanes that are currently flying are running an ignition battery eliminator such as the Tech Aero IBEC. I personally haven't run a seperate ignition battery in over a decade. Gas engines also do not require any break in running. Once the needles are set well enough for the engine to transition from idle to full throttle it is advisable to start flying the engine and tune to what is observed in the air. Bench running will not provide enough cooling and damage can result.