What size motor would be best (or do you have) for ...
I would undoubtely stay fixed to the 15 HP outboard.
Switching to a 10 HP will not make any change, since it weights the same as a 15 HP. And even a 6 HP, or a 2 HP outboard could move your boat, that is true in a quiet sea with no wind. In a choppy sea, aginst wind and current, your 6 HP outboard will hardly motor windward, if not backwards.
In this case you will need a long shaft. Short shaft are designed for dingies.
If you want to get rid of your 15 HP, I would rather buy a pair of oars instead a 6 HP.