Selecting the perfect fly rod may not be as easy as it sounds. There are many factors to consider and what anyone tells you may not be the best for you. Below, we will try to present some of the reasons for different choices, but ultimately the selection of a fly rod is a personal choice and the performance must meet your personal requirements.
Favorite Destinations
One of the first considerations in the selection of a fly rod is the type of terrain and size of the stream that you fish. Obviously, if you fish the tiny “blue line” streams, you would not require a heavy weight rod. An eight or nine weight rod would be unsuitable.

8 foot, 4 wt. Winston and 8 1/2 foot, 5 wt. Sage
Possibly, a shorter rod, say seven feet, in a three weight with a small reel would be entirely adequate. Some seasoned anglers even use lighter equipment as a personal challenge.
By the same token, if you fish larger water and the fish are sizeable, you might need to move up to a 7 or 8 weight rod with a compatible reel. The high end of the spectrum may not be required unless you are after heavy warm water species or trophy water trout.
Another consideration is your budget. If you fish various sized streams and you don’t wish to spend big bucks, you might go with a five weight, which is a good compromise. A five weight, 8 ½ foot rod will suffice for a wide range of stream sizes, as well as being useful for warm water species.
Additional Factors
There are many experts who advise using a shorter rod for small streams, but in some instances the longer rod is more useful. A good example is when you are fishing small pockets of water. A long rod will allow for “dapping.” That is a technique where you don’t have room for a long drift, but merely extend the rod over the pocket and “dap” the fly on the water, much as a hovering insect that might be ovipositing. I have found this technique to be a good practice on pocket water.

8 1/2 foot, 7/8 wt. St. Croix and STH Reel
The size flies that you intend to cast play a role in selecting a desirable rod weight. Heavy weight flies might place an undue strain on a light rod. Likewise, a tiny fly selection might be difficult to cast with the light presentation required for such an offering.
Reel Selection

Assorted warm water reels
When choosing a rod, it is important to select a reel that is compatible with the rod weight. If the reel is balanced with the rod, they will function as a unit and casting will be more pleasing. If a small rod is over-reeled, the unbalanced unit will result in a day of frustration on the water.
Final Word
A fly rod is a very personal tool and must suit the user. I would advise, if you are new to fly fishing, that when you are considering your purchase, you go to a shop that will let you cast several weight and length rods to find one that you feel comfortable casting. After all, you are trying to please no one but yourself.

Reaching way back when only fishing for warm water species with cheap reels and fiberglass rods
As you progress, you will find that pressing need to purchase multiple rods, reels, vises, etc. That is the curse of the fly fisherman. We all need more “stuff.”