Big enough to not fit in your largest fish's mouth. I know, it's the obvious answer...but I haven't been able to get a better one, I asked the same question. As long as they can outrun the adults that's fine. When I dropped my Molly in the tank with the guppy fry, they all were smart enough to run for the hills at 1 week old. If he gets eaten, don't fret, you'll have more soon!