Rule of thumb is that if it flies well with an Arden 19 or Ohlsson 23 - think more like a Enya 09, or for a power boost, a Medallion 09. Any of the baffle-piston 15s (with the one exception, which the foundational principles of The Hurl prohibit me recommending for this application) are generally plenty and will give you a great power boost compared to the original.
The problem with these sorts of models is that for an Arden 19 or similar, they have to be tiny for decent performance, but they are build very heavily to take the vibration. So something like a Tee Dee 049 would get you similar power, but the rest fo the model is so heavily built that its a brick anyway. It might be the right size for a 1/2A, but way too heavy. The SIG Akromaster is a good example, its' smaller than the best 1/2A stunt planes but 50-100% heavier.
Brett