I will say at 2in at least. Big enough so they dont fit in their mouth. Before putting them in make sure they are well fed & as you are feeding them put the in.
I put in my yellow lab in when they were 1in. So far they are fine with the other guys in the aquarium. (130) 21+6 juvi. Two of the Juvi have now started their I am boss dance & even locked jaws for a sec. Right now I have another female holding in another aquarium that I took out.