If you were lucky enough to have a sales person match the reel with your new rod, half the battle is over. The fly line is almost as important as the rod and reel. If you have the wrong weight line on your new reel. Forget about learning how to properly cast for distance and accuracy. The fly line must be the same weight as the fly rod and reel.
Lines come in two distinct variations.
Dry fly surface fishing is great fun its exciting throwing flies at rising rings left by hungry trout hitting bugs on the surface. But, in reality most all fly fishing is done sub surface, often down deep where fish are holding and resting. To be perfectly clear on several important points mentioned above. The length of your new fly rod is really up to you and your ability to effectively handle , cast, control your new rod. A whole column could be dedicated to just the aspect of length and cost alone.