I don't have a "required" pounds/strength answer for you, but the usual cord for a ridgeline is 1.75mm or 2.2mm LashIt or Zing-It. Lashit is usually gray; Zing-it is usually yellow. The 1.75mm size has a breaking strength of 500 lbs; the 2.2mm size has a breaking strength of 650 lbs.
The cord loops are woven, not knotted. They can be cut to a fixed length, or made to adjust by burying (weaving) the end of a loop, or the second cord if using a two-piece design, inside the other. That feature gives you an adjustable cord. The downside is you have an adjustable cord - that may "adjust" by itself if it loosens up when the bury is not under tension.
Think in terms of a structural ridgeline vs a removable ridgeline. The fixed ridgeline takes the hang stress from the suspension webbing/cord/daisychain, etc., so the hammock hangs peacefully, with the same sag each time (assuming the hang distance is long enough that the ridgeline is just snug), regardless of the hang angle.
The removable ridgeline can be any light cord, even a string. It is just a "yardstick". You attach it to let you know when the ends of the hammock are at your desired distance, then remove it (or at least one end if you want to keep it attached to the hammock so it doesn't ... wander off).
Then we come to the do I set the distance when I'm inside the hammock or outside. This falls into the category of "discussions that go on forever." I'm not in the hammock when I'm setting it up, so I'd size my ridgeline for the desired end-to-end distance when I'm outside the hammock. And I'd test it by getting in the hammock, grabbing the line, and trying to give it a slight bend. It should not be guitar string tight. There are many YouTube videos about this if you search "Hammock Ridgeline".
