Fly rod sizes are categorized by numbers. Unlike flies, where the smaller the number, the larger the fly. Rods go by the larger the number the larger the fly rod. A number 3 rod is a very light rod, used for small trout in small streams. A 12 wt. rod is used for Tarpon, Stripers and other fish that need long distance casting, big flies. The most popular all-around rods are in the 5-6 category, here you can cover the whole gamut of trout fishing and a good chunk of bass fishing as well. A number 6 rod can also tackle small steelhead and other sea-run fish, if the angler is skilled. Most steelhead anglers will choose a bigger rod however, in the 7-9 range, and they can also fish salmon with that range as well.