There are only two possible people it could possibly be.
One is Bob Kirkland and the other is Brett Stiles.
These are the only two suspects that are under 6'. All the remaining suspects are over that height and the blind girl said that Red John was under 6'.
We know that it is not Sheriff McAlllister from the last episode because he is not afraid of heights and Red John is. Once we determine which one of these is or is not afraid of heights then we will know who Red John is. I have a tendency to believe it is Bob Kirkland because Jane said that RJ was a member of Stiles group (Red Barn) as a teenager which would make him about Kirkland's age. Stiles is in his 70's and too old.
