Of course it's theoretically a difference if I've a 4" or 5" fetching. I wonder if it's really practically noticeable? I'm asking from a bow-hunter's point of view (strong bow/powerful arrow).
Fletching is used to stabilize an arrow, stopping it from tumbling. A heavier arrow needs more fletching to help stabilize the arrow. This is usually accomplished by lengthening the fletching rather than broadening it.
However more fletching adds drag, and it also has the potential to add more error into the mix, as a beginner you should use arrows that are recommended by your outfitter. As you gain more experience, you can start to tune your outfit to better suit you, your styles and skills.
The questions you have been asking here are all well and good, but they and their answers are all theory until you have some good practical experience.
It's smart to get as much info as possible, but in the beginning you really need to trust your outfitter. What you learn here can get in your way as a beginner.