Generally you can get away with 1" less length in feathers compared to vanes for "Steerage". More drag and quicker recovery with feathers apples to apples.

For mech heads, 3"vanes are OK, and 3" feathers are great, epecially in full helical (they clear fairly easy even on a non-dropaway rest). I shoot 3" feathers in a RH for some of my set-ups. And like Mike said, they are much quieter.

However, For fixed blade heads, definitely the bigger the better. Amazing what 5" offset or helical feathers can do for unruly or high speed fixed blade broadhead flight.