I think my 12 footer is rated for a 7.5. I use my 2 hp, 4-stroke on it. I have had a 5hp mounted on it and wouldn't want much more. The bigger motor eats away at weight capacity (fishing lures, lunch, etc) and can make things quite scary. Since you won't be taking the boat on any big water, there's not really a need to go over the rated HP.
My $0.02