I had built two Royal B-17 model in the past and had started a third that never got done (it got destroyed in moving a long time ago).
But they all sounded awesome with four .20 size two stroke engines on them.

I agree as the engine gets smaller the RPMs go up and the less realistic they sound. So a 5 cylinder radial would tend to sound more like a double row or triple row radial does.  Anyway four 5 cylinder model radial engines would sound prettry awesome in a model B-17 too.