Re: What is the longest stock for AR?
If you're using an ACOG or red dot optic, the "nose to charging handle" head position reduces the need for a long LOP. I'm 6'4" and usually shorten the LOP with an ACOG and my nose almost touching the charging handle.
If you run out of LOP with a longer scope, try moving it further toward the muzzle. With a scope such as a NF 2-5-10x, I position the end of the ocular just above the end of the charging handle.
If you're tall, it's not the length of your arms, but the distance from your shoulder where the buttstock touches to your eye that counts. You can also work on positioning your head to alter this distance and reduce the need for a longer LOP. ARs seem to be different in this respect than bolt action rifles.