Ok one of the clues states that Red John is "just under 6ft tall." That being said i took the list of seven suspects and looked up all their heights and here is what I found.
Brett Stiles- 5'8.5"
Gale Bertam- 6' 3"
Robert Kirkland- 5' 9"
Sheriff McAllister- 6''
Brett Partridge- 5' 11"
Ray Haffner- 6'1"
Reede Smith 6'3"
If you go by height alone you can rule out Bertram, McAllister, Haffner and Smith. That leaves Brett Stiles, Robert Kirkland and Brett Partridge.
Next you can rule out Brett Stiles because of his age leaving Robert Kirkland and Brett Partridge.
I think Brett Partridge is Red John simply because he and Jane have never liked each other.