The answer to that question depends greatly on the type of fish that you are trying to catch, as well as the rod you are using, and the fishing situation that you find yourself in. First, you’ll want to get a line weight that can be handled by your rod. So if you have a rod that can handle a 5 lbs weight line, you’ll want to get a line that is between 4 and 6. Also, you’ll need to take into account how you cast. If you’re using short quick casts, as is often the case in rapids, a shorter, or smaller weight line may be better, since it can quickly and easily dart above and on the water. Longer, more drawn outcasts, as seen on lakes or calmer rivers may benefit from long lead lines since flies often spend more time on the water overall in these situations.