If time is important to you, then the size and power of the saw should match the task at hand. If you are just cutting a few cords of wood here and there, a little 40 or 50cc saw can do just about anything you ask of it, if you aren't in a big hurry.
I could outfit one of our smaller 50cc saws with a 20" bar, and get through just about any job we encounter, as we cut mostly tops. Even so, we have sized up the power of every saw in our line-up, and put the approprioate size bar and type of chain on it accordingly.
Our 480CD is fitted with a 24" bar, 3/8 full chisel, and is for the most part used only for material apprx 18" and larger.
The 268XP is fitted with a 20" bar, 3/8 full chisel, and used mostly for larger material as well.
The 262XP is fitted with an 18" bar, 3/8 full chisel, and it a workhorse of a saw for material from about 10-18". Relatively light, and very, very fast cutting.
All of the 50cc saws we own use 18" bars .325 semi-chisel chains, and are used the most. They are light, fast, easy on fuel, and easy on my back now that I'm 50 years old.
The CS-370 and CS-360T are used for small stuff, or anytime I've got a LOT of small limbs to trim from the trunk of a tree, or a lot of walking around to do cutting small limbs from a top.
For the most part, we size the saw to the work. Each saw has been outfitted with the bar/chain combo that keeps it easily in it's power range, and minimizes the time required to make cuts. This keeps from working both the saws and the operator any harder than needed. I don't head out to the woods just to listen to saws run, I want to get a LOT of wood cut up, efficiently and quickly, as the hardest part of the task is splitting and loading it up.
I've read on here and have seen a few folks outfit their saws with relatively long bars for the cc's of the engine. For example, we have used several bars on our 262XP, and consider the 18" bar the perfect choice for it, but it could easily run 24 or 28" if needed, and there is certainly nothing wrong with doing so, if it saves someone from having to buy another larger/more expensive saw.
Basically, it all boils down to time, don't expect a small saw to really shine in large diameter hardwoods, no matter how long of a bar you outfit it with. Plan on being on that great big log for an hour or more just to get it cut up with your little 50cc saw, when you could have came in with an 80cc saw and cut it up in minutes!.......Cliff