For power plants, it is a no-brainer. You want 1HS plant and that's it. Then you just put enough of them on a ship to ensure that the loss of one or two doesn't cripple your fighting power. There is no benefit in designing larger power plants, it's just a waste of RP.
For engines, pretty much what others said. A single big engine can act as a pretty effective armour, since it has lots of HTK and can thus tank several low-damage hits without dying. Regardless, you always want at least two engines in a military ship for redundancy. If you only have one engine, and it gets taken out, then your ship is stranded. Same with fuel tanks - even if you use a single Ultra Large tank, always put in one Small or Tiny tank in case the main fuel tank gets taken out. Larger engines also have better fuel economies and despite what davidb86 said, fuel can be a major concern depending on how your empire grows, so it's not always a moot point.
And yeah, for commercial/civilian designs, you want as big of an engine with as much consumption reduction as possible. Even if the ship moves at under 1k kms, because those freighters will likely be constantly on the move. Shaving off few days or weeks from their schedules is generally not worth it when compared to the amount of fuel they consume when moving 24/7/365. You might want to build a fast freighter for strategically important logistics missions, I generally do so at Ion level, and those few ships don't haul stuff around constantly.