Pretty much agree here...

For commercial ships I have some few which basically use the fastest .5 power drives for strategic reasons. These ships will not be running normal transport operations and are generally reserved for military logistical purposes or fast transport ships part of the military branch.

Since I usually start at conventional tech levels there is a huge abundance of wealth for the better part of the start of the game, this wealth are used to feed the civilian shipping lines who are the main haulers of stuff for the most part. Civilian ships only require wealth to operate not fuel.

I only concentrate on building the important stuff, not regular transports or colony ships.

Ships like mineral haulers, tractor tugs and the like get the most fuel efficient drives I can make.

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.

I am not convinced on the redundancy issue.  Higher HtK may keep the engine from being destroyed in the first place.  For small ships, I look at the odds of it avoiding a mission kill from damage, rather than trying to preserve minimal functionality.  I would rather have a small chance of it maintaining full speed, and not have to break formation, than have it be guaranteed to lose engine(s).

There are a few things to take note of here... redundancy is that the ship can at least move even if at less speed and it can be repaired more quickly than if totally lost. If the engine explode it will do less damage too.

Two engines have the same HTK as one big engine so you simply diversify the risks.

And as Steve said... in C# Aurora bigger engines will have less HTK than smaller ones so there you actually take a much greater risk. In C# Aurora you definitely want at least two engines for that reason alone I guess.

I am not convinced on the redundancy issue.  Higher HtK may keep the engine from being destroyed in the first place.  For small ships, I look at the odds of it avoiding a mission kill from damage, rather than trying to preserve minimal functionality.  I would rather have a small chance of it maintaining full speed, and not have to break formation, than have it be guaranteed to lose engine(s).

Yes... Power Plants in VB6 Aurora don't get better efficiency because they are larger but they will in the next version. So 1 HS Power Plants is enough.

As for engines there is some merit to having bigger engines but on average you are better investing RP into fuel reduction technologies than building extremely large engines and then have to design multiple types of engines for every tech level you come across.

In general the higher your engine and fuel efficiency technology is the more economical from a RP consideration it is to design a big military engine because it takes less and less RP in comparison to researching the next level.

For civilian engines you just design the biggest possible engine because they are so cheap to design and build anyway, good for your fuel economy.

In C# Aurora there will be a much bigger difference of fuel efficiency of small versus large engines so it might become more of a sound investment to design big engines even early on, but never go nuts about it and design one engine for every ship type you use.

When it comes to engines I like to make sure ships have at least two engines for redundancy, engines also tend to be the most expensive components on most military ships so they also dictate the minimum of supplies the ship need to repair them. You need twice the supplies to repair combat damage, this make really big engines a somewhat liability for cost savings and need for more engineering sections on some designs.

Yes.

Or rather, it depends.

Larger engines and power plants are more efficient, but lack redundancy.  Multiple smaller systems ensure you don't lose all your capability to a single hit.  Custom-designing systems for each and every class requires a fair amount research.

What is 'best' for your empire depends entirely on how your empire defines 'best'.