When I used to use minnows I always preferred using the smaller ones even with out any evidence that it made a difference in the size of crappie I would catch.
One thing I really took notice of, now that I mainly use plastic, was the four inch or larger shad I sometimes would find disgorged in my livewell and this sure convinced me that I could use much larger plastic baits while fishing for crappie and in so doing I am now amazed at just how small a crappie I would sometimes catch on the larger baits.
And like other issues as to whether size matters I guess the old question is still unanswered even for crappie fishing with minnows. :D
“There is no difference between communism and socialism, except in the means of achieving the same ultimate end: communism proposes to enslave men by force, socialism—by vote. It is merely the difference between murder and suicide.” Ayn Rand