The fly fishing rods are made in different materials: bamboo, fiberglass, carbon fiber; as well are made in different lengths. The length depends on where you are fishing and which is your fish target, it can be from 6 feet up to 10-11 for the single hand, and up to 13-15 for the double hand rods.
Here we consider the single hand rods.
Shorter rods (6 to 9) are ideal for small streams and rivers, where even the fish supposed to be smaller.
Longer rods (9 to 10) are excellent for larger rivers and lakes, where you can find the big predators.

When you buy a rod you have to consider also the line weight, it is measured in grains, from 1 to 14.
We need a separate post to talk about all the different type of lines (floating, intermediate and sinking) and weight, just to know that is important to match rod, line, and reel.
The manufacturer writes in In every rod which lines match, as you can see in the picture
How you can see the topic is various and would take a lot of time to go in every single aspect.

Now probably you are asking to yourself: “So, which one is the best rod start fly fishing rod for beginners?”
The best rod for beginners is a 9ft #5-6, with this kind of rod you cover almost everything, from high stick in a small creek to cast medium-large streamers in a lake or in the sea.
A universal rod, a perfect rod to start get into fly fishing, then when you realize if this sport is for you or not, you can pass to other different rods according to which species do you want to hook.