Welcome,
"Optimal" is a choice word for engine size on a sailboat.
Decide on how you want to mount/use it: always hung off the stern, side mount, removed for racing, in and out of a tight marina only, port to port on a long quiet sound or bay, etc.
If you plan to regularly remove it, smaller is better--3 hp.
If you plan on using for longer runs, and leave it on the stern, up to 6 hp. Anything more is overkill.
The electric torquedo's are nice for compact size, thrust, and quiet.
Nice question, come back to the Herreshoff Forum as you progress.