There is only one size for a thruster, bigger.

Due to various installation constraints, very few vessels can
accommodate a thruster large enough to be useful when you really need
it. It will be a convenience when you could manage without it but
won't have the umph to bail you out of a tough docking situation.

Figure out what the constraints are and then put in the biggest and
most powerful one you can. Note that any tunnel thruster large enough
to be worth paying for will create a lot of drag on a sailboat. They
don't go straight through the water when beating to windward and the
pressure differential will cause cross flow. Water flow in the bow of
a boat is very laminar and anything that creates disruption will
create a lot more drag than if it was farther aft.

--

Roger Long