Size
As mentioned above, size falls under the umbrella of silhouette when discussing fly pattern design. Here, I will discuss size more specifically. Analogies between human actions and the actions of trout are typically helpful in the study of fly fishing, so let’s remember our hamburger and chicken sandwich analogy. If a person orders a Hamburger, and gets a chicken sandwich, they might take a bite before they even realize that they got the wrong food. However, if that same person were to order a hamburger, and received a hamburger that was 1 inch tall and 1 inch wide, they would immediately notice. Additionally, If they ordered a chicken sandwich, and received a chicken sandwich that was 3 feet tall and 3 feet wide, they would also instantly notice. If a trout has been eating size 22-24 midges and Baetis in a tailwater for its entire life, and it sees a size 8 blowtorch, it might not even register it as food. Sure, it might take the pattern out of curiosity, but it likely isn't the best choice for an angler looking to maximize success. In fly tying and consequently fly fishing, size really does matter.