Originally Posted by MadisonBoats
By size; I mean what the fishing are currently taking in the location you are fishing. The difference in hook size vs. what size the fish are eating is huge. You should find a simple seine net online or at a hobby shop that you can seine the water you fishing. This is a great help in identifying the color, size, and bug that is prevalent in the stream. Save some of them in a small watertight canister and use as reference at home - by copying them.
Along these same lines it is also important to understand that just because a hook is size 12 doesn't mean the fly is a size 12. When I say they were taking a size 16 sulfur, that is the size of the insect (or fly body) that the fish are eating. A TMC 100 size 16 is completely different from a TMC 2488 size 16 or a TMC 2302 #16. It is important to know how to relate the insect size to the hook shank length not the hook size.