size is going to be a function of how and where you use the boat. If you operate the vessel in an area with strong currents or winds you're going to want more horsepower. You can get by with 5 hp, which will probably get you up to hull speed
in flat water
, but you'd be better off with a 9 hp if you are in challenging conditions.
Regular or long shaft is going to be a function of how and where it is mounted. If you have the option, I would mount the bracket higher and opt for the long shaft.
Not sure what you mean with your last question. Can you be more specific?