It grows. Simple.
On the moment it's a lie, since the nose had no intend to grow in the next minutes. The fact that he said that makes it a lie.
Yes, consequentially, the prophecy realizes itself. But it doesn't matter anymore. The lie was already there.
If it's not true, you could use this loophole to basically... predict the future.
Or the nose grow when Pinocchio *intend* to lie, by omitting or denying the truth. There the nose will grow depending on which event Pinocchio is convinced will happen.