What does your plate say?How wide is your boat? I personly know that a 9.9 Merc. 2 cycle has enough power to flip some lightly loaded 14' Johns in a wind gust.Also,where are you going to be using it?Many places across the country now have a "under 10 hp.limit".
Please remember.Too big a motor has killed far more people than not enough motor.
In a quick lookup on line,it varied between 6 hp.and 35 hp!
The weight of the motor has to be considered.
If it is to be tiller operated,the weight of the operator and the weight of the motor all in the stern has to be considered.
You can not trust the word of all dealers as they are not liable if they sell you a motor alone and do not mount it.The larger the motor,the larger the profit.I have seen this done quite a few times in my life!
If you have a commercial fishing license,you may not be limited.
I was raised with a 2 hp.Evenrude on a actual 1 " cypress planked,16' long,6' wide 3 ft.sided Oyster Bateu that probably weighted around a ton when the wood was soaked.The transom had to be cut out to 15" for the Evenrude as a Old British Sea Gull Sail boat outboard had been used.This did fine for a child on the Santee Lakes of SC.It would have been useless in a fast river current.
Check the maker of your boat if they are still in business and your states fish and Game department may be able to help.
I have a 14' long x 48" wide John & a 14' x 57" wide John.Both did fine with the 6 hp.,8 hp. and 9.9 hp. motors I have had on them.The 9.9 Merc is too much motor wide open with someone under 150 lbs.in the 48" John in my opinion.The 48" is too narrow for my 9 hp."Go Devil" outboard in my opinion.It does fine on the 54".The high center of gravity is the problem.Common sense and it is no problem.Some folks do not know that you can slow a outboard down.
I have known people like me who operated boats and motors for 50+ years to finely make the one mistake that it took to kill them in a boat.
I love you Brothers and Sisters.peewee