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?