Point 1) Sometimes 2 is enough, sometimes 4 is enough, really just go with whatever satisfies your eye test
Point 2) Longer track sections are fine, and even sometimes preferable to smaller ones. I myself tend to use 5 or 6m when doing larger rides like Hypers or Gigas, and I use 4m for smaller rides or inversions. Honestly this is also an eye test judgement you make yourself too, it largely just depends on how big your building really!
Just have a mess with it, after enough tries on different coasters you'll pretty quickly grab the hang of when and how to cut corners with this kind of smoothing, I know one of the biggest gripes with this is how long it takes, but its for sure worth it to some people!