If you're going to shoot any sanctioned "Hunter" class IBO shoots you NEED 4" fletching.
You can use short stuff in any other class.

I suggest vanes because they don't drag as much, and if you plan to use 4" my next suggestion is the lighter "Low profile" 4" style that you can get from Duravanes, or the tougher more expensive FLP400 Flex Fletch.
They are lighter, less drag prone and behave much like a smaller vane yet conform to the equipment regulations.

If you plan to shoot a more advanced class like MBR, Open, etc small vanes are the rule. Duravanes, Flex Fletch, Bohning X-Vanes etc.
small stuff for skinny arrows, on up to something in the 2.25-2.5" range for the fatter shafts.