The rule of thumb is when they are too big to fit in the largest fish's mouth. Sometimes, you might want to wait a little after that, just to prevent bullying a little more, if you have semi-agressive fish, or fish you think might pick on them.
I would also see to it that the fish are well fed just before you put them in. I would also put in one and see if she makes it for a few days before putting in the rest.