elknutz has it right.
Lots of options and lots of fun...and you can't really go WRONG unless you stick on really tiny feathers with a broadhead.
but you don't HAVE to go shorter.
You can just have another 5" or even 5.5" if you want...more steerage.
What I'm going to say might be a bit of hair splitting but I like that stuff....
More than just total length of feather, I feel the total surface area is a better way to compare feather profiles and setups.
Kinda tricky to do though, I just take a pic of the feather and measure the area on photoshop...I know..I know CHEATER right?..lol
to take it to extremes..a 6" feather that is only .25" high has a lot less surface area than a 3" flu flu..see what I'm getting at..?
What I like to do is find a nice balance using a quiet profile like a T-hawk, shortening it a little bit (4 5/8") while keeping the same height( diagonal base spacer in the chopper).the shorter length makes it even quieter.
And end up with four really quiet fletches that have a little more surface area than 3 5" shields.
nanners are a bit quieter, but I prefer to have the tall part of the feather that does the most work closer to the rear of the arrow.
..like I said nit picky..and unnecessary...but I like that stuff.
That's my psychosis and I'm sticking to it.