I appreciate the detailed answer but that wasn't really my question. I wanted to know, in regards to material science, how much stronger we would have to make a regular fishing line of typical diameter using a future material, not how big a line we would have to use. Would it have to be twice as strong? Ten times as strong?
> I wanted to know, in regards to material science, how much stronger we would have to make a regular fishing line of typical diameter using a future material,
That line already exists today. Fishing line is measured in breaking strength, so a 10 lb. line should reliably be able to withstand 10 pounds of force before it snaps. (In practice, it's less because knots are always the weak point. Also, it's a gamble as to whether a manufacturer accurately measures their line's strength.)
Gravity exerts a force in the same unit, so if you want a line that will let you lift a 50 pound fish out of the water, 50 pound line will do it. Maybe 60 or 70 to give yourself a margin of error and account for the fish's thrashing. The line will be somewhat thicker than lighter-weight line, but not enormously so.
So the literal answer to your question is just: Go to the store and buy a line whose test weight is somewhat above the weight of the fish and you'll able to muscle it onto the boat without having to tire the fish out first. Your local fishing store will have line heavy enough to do that for almost all fish.
The obvious follow-up question, then, is, if you can just go to the store and buy that, why don't anglers do so? And my comment is mostly an answer to that.
