My female platy gave birth about a month ago now and id say each are around 1cm long, i was just wondering how long they need to stay in the baby tank for, there is one baby bigger than the others about 1.5cm long?
Once the fry are large enough so that they cannot be eaten by any of the other fish there, it is safe to move them.
But, one other thing to keep in mind is that some filters have stronger suction than others and you will want to be certain that the fry are large enough to be able to swim away from it.
The suction thing depends on the model and tank size, of course. My newborns had no problem getting around my 5-15 gallon power filter.
The not-fitting-in-the-mouth rule should work. However, some fish might still nip at them and stress them out. The bigger one is getting more food than the others, so you might want to remove him before the others to make sure that it doesn't hog all the food.