I hate to be the Devil's advocate, but one of the best matches to a 20" bar that was ever made was a McCulloch 610 or 650 TimberBear that was first built in 1977. The engine is about 61cc and it pulls a 20" bar without breathing hard. All three of the ones I have in this class (including the 5700) are equipped with 20" bars and they still cut like bandits.
That being said, I truly like the way my Stihl MS361 handles a 20" bar and it proves it every time I go to the woods or to a GTG. However, any bar longer than that and it has trouble and will not even keep up with the old Macs pulling a 24/25" bar. The heavier old Macs can even pull a 28" bar easier than the 361 can pull a 24/25". Hard to believe, but true.
That being said, I truly like the way my Stihl MS361 handles a 20" bar and it proves it every time I go to the woods or to a GTG. However, any bar longer than that and it has trouble and will not even keep up with the old Macs pulling a 24/25" bar. The heavier old Macs can even pull a 28" bar easier than the 361 can pull a 24/25". Hard to believe, but true.