Rarely do I hit the water without at least two fly rods, regardless of the conditions
Admittedly, my favorite fly rod is a 3 wt. My dad gave it to me for Christmas when I was in college. I own rods that most would deem as "nicer", at least based on price, but it casts smooth and can lay a bug down as gentle as can be. Rarely do I embark on a trout fishing trip without it. But when I'm chasing bigger fish with bigger flies, it is a no-brainer to leave it at home.
To me, choosing the right fly rod weight is about five things: fly size, fly weight, line weight, weather conditions, and target species. The most important factors are fly size and weight. Even with perfect form, throwing big flies on heavy rods will tire you out. I often refer to a story my good friend and expert fly angler Levi told me. Before embarking on a trip south to catch tarpon and other big game species, his would-be guides recommended that he learned to cast both left- and right-handed, for the simple reason that you are bound to get tired chucking big flies long distances for any period of time. Throwing big flies on undersized rods not only tires you out a lot faster, it often leads to poor form, sloppy casting, and sore arms. Fly anglers are already susceptible to overuse and repetitive strain injuries, which are generally associated with poor mechanics and kinematics. The bottom line is this, if you plan on throwing big flies for the species you are targeting, go to the high end of the recommended rod weight for that particular species. For example, most companies recommend a 3 wt to 6 wt rod for trout. Although I use 3 and 4 weight rods most often, I use a 6-7 wt rod when throwing big trout streamers and mice patterns. Throwing those big patterns on light rods gets frustrating quick, to the point where it isn't nearly as fun as it should be. If you aren't having fun on the water, something needs to change. Thankfully, I am seeing more charts that recommend fly rods based on fly size, rather than the conventional fly rod-target species charts. For example, this article and chart from Back Country Chronicles are pretty informative.
Line weight is another important factor. Generally, the line weight should match the rod weight; however, some rods cast better with over- or under-weighted lines. Additionally, sinking lines are often listed by grain size (e.g., 300 grain line) rather than traditional line weight. So, it may take some trial and error to find the right line. Here is a really basic, but useful line weight chart.
Bowed up on a 3+ ft gar a few years ago
Weather conditions should also be considered, especially when fishing bigger, open water. A few years ago Mary May and I were fishing a high-alpine lake in Wyoming. The wind was gusting and you had to time your casts between gusts to have any shot of getting your fly out far enough to get a bite. We were throwing small ant patterns, but it felt like I could barely get my 3 wt off the bank. Again, I was starting to loose my cool. I decided to pick up a 5 wt just to see if it made a difference....mind blown! Suddenly I was getting my fly three times as far off the bank and into areas with active fish. I'm not sure why that was my eye-opening moment, as its not rocket science, but it definitely sunk in. Now, I always check the wind before heading out and pick my rod accordingly.
Finally, always pick your rod based on the target species. This is generally how most companies and sites categorize rods - by species. These references provide great basic guidelines, but they are just a small piece of the puzzle. For example, here is a pretty standard chart published by Cabelas (the first hit via a Google search).
So as much as we all want to say that we caught our trophy on light tackle, consider all the factors before making a final decision. It will make your day more enjoyable and take a much lesser toll on your body! Tight lines!