I don't think it's so much the size as it is the developmental stage where the coral begins to take on its' growth form. For instance, sometimes you'll see someone's Green Slimer shoot out into odd staghorn forms that stretch toward every side of the tank. Then in other tanks you'll see it actually table out a bit and grow somewhat like a bottlebrush. Aside from lighting, I believe there may be a link to the flow pattern provided after the coral has established its' form that may help cause its' demise. Sometimes I've seen the same instance when I wanted to add one more frag in the path of a well established colony and then all of the sudden seen the new frag take off while the other one developed a problem from being blocked by light, change in flow,
The reason I asked the size is due to past, personal observation.
There used to be a dealer called reefermadness.
They sold a lot, mostly all, wild colonies.
There was a 50/50 survival rate over 6 months IME.
However, we noticed that if we fragged the colonies into chunks, not small frags, that the survival rate went up dramatically.
The only thing we could reason was flow.
In the wild the flow is tremendous, in our tanks not nearly the same.
For an animal that eats, breathes, and poops out of the same hole...flow is important.
When fragged down, the corals would grow into their environment.
Eight years later I still have a stag from reefermadness.
Hth,
Sean