How big should fish be not to get ate by adults?

[Art]

I have a large tank 180 gallon with adult tilapia fish.  I rescued babies and was wondering how big should the younger fish have to be before they get ate by the adults?  or do they have to be adults before I reintroduce them back into the main tank?

How long do adult tilapia fish live? when should I harvest them before they die of old age?  currently about 1 year old.  I would prefer to harvest only 1/2 dozen at a time. 

[Randy]

I would say 2". The small fish I have put in my system stay near the over flow plumbing even though it is in the end claimed by the dominate male. So I would recommend you provide protective cover for them.