To a novice fly angler, the fly line size system used in fly fishing is often a confusing jumble of numbers. What fly line size do I need for trout? Can I fly fish for any other kinds of fish with my trout rod? What size flies can I use on my rod? Because the fly line size is the first thing you should decide when picking a fly fishing rod, and it will determine what you can do with the rod, it's a pivotal decision.