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.