How do you know what is the correct horsepower outboard motor for a boat?

All watercraft come with a BIA rating certification label affixed to the craft. This label has such information as horsepower rating, and passenger/load limits printed on it. If by chance the label is missing on the boat in question, check a similar rigg, eg. same size and construction material, for the information. A trip to a local marine dealership is good place to start.