Anything taller than 5'7 is plenty big enough to play an outside Corner spot, Amik Robertson was 5'8 & was complete bulldog playing Field Corner for La Tech. He had 60tcks 16PBU's & 5INT's his Junior year last year & got drafted in the 4th round.
Couch is legit 5'10 with crazy long arms, he's skinny & on the slender side but he's not too small to play Outside.
They don't play the younger Corners because this Defensive staff always defers to upperclassmen, they don't care how good a player is, if he hasn't been in the system for at least 2 years they're not gonna play them unless they have no choice due to injuries.