weigh them. If they were rectangular so every portion of that 3 or 4 or 5 inches is the same height, then you could easily know, but they are cut. A lot depends on the style and depth of the cut as to whether they have the same "wind catching" ability.
That said, I have experimented with different sized feathers and find that for field points I couldn't tell the difference. A big broadhead MIGHT bring out a difference.
You also might consider the immediate stabilizing effect of large feathers and the lack ( relatively) of wind catching ( side wind) on the smaller ones. I have had heavy side winds blow my arrows almost sidewise on a shot numerous times.
ChuckC