Question On Fish Stunting

[Seed_Lord]

Hello, basically my brother fishes a private lake for carp, hes had the idea of when the carp spawn taking some fry and tank/pond rearing them till they are bigger then putting them back, providing the lake owner is ok with this, question is how big would they get before they'd have to be transferred into a pond? with a 50 gallon tank being used before any stunting could occur, not planning on growing them too big, big enough so perch and small pike wont be able to get them

cheers for any info