I've never seen western tack advertised as "cob" or "full." Those are English terms and most western tack is sized for Quarter Horses, so unless he is unusually small or large most western tack should fit your Quarter Horse.
Now I used to have Arabians and I was constantly punching holes in my breast collars and headstalls to make them smaller. But you should not have that problem.
PS. Cob sized English bridles used to fit my Arabians perfectly, so if I was picking English tack for a QH, I would choose "full."