Your question is timely. We got a 5hp (equivalent) Elco electric motor. We are finding the 5hp to be underpowered for our conditions. We keep our boat on a dock on a marina that is about a mile down a tributary of the Hudson River, so we have a bit of a hike to get to the Hudson to sail. The Hudson also has strong tidal currents (up to 2-3 knots). It moves the boat, but slowly (if we get it up to 3 k/hr we are flying and we only do that under good conditions) and under challenging conditions we don't have a lot of power.
We are seriously thinking about upgrading to a 9hp and that is what a lot of the boats that are our size on the Hudson have. We went with the 5hp because of battery cost/weight and the positive is we have a 100ah lithium battery that weighs 40 lbs and holds enough charge for several days of sailing with a lot of motor use. If you are only using the motor to get out of a marina and aren't fighting tides a 5 hp motor is probably fine, and from a battery perspective (if you are considering electric), much more economical. But if you will be motoring a distance to get from point A to point B and don't want it to take forever, or need to fight tides and currents you probably want something more.
That said we do like the electric motor a lot. Easy to start, not heavy, reliable, and quiet.