Are we talking motor or engine?
You have the cart before the horse if I understand your question. Motor design is based on application. Physical consideration cannot control design. Application is what will determine it's horsepower, ambient temperatures, loading, it's design rating, etc. If you can be more specific re: your application, I can help you.
Other people asked questions on similar topics, check out the answers they received:
Other people asked questions on various topics, and are still waiting for answer. Would be great if you can take a sec and answer them