How old does she have to be to tell the judge that she wants to live with me?

I don't think you provided enough information for a reasonable answer

I'm sure with more info there are many answers for you

Most courts allow a child as young as 8 ( some younger) to testify in court.

But no matter how old, it is not just a matter of what the child wants,

You will have to file a motion in court asking for a change in custody. You will present other proof that the child would be better off with you, and the child gets to testify. Then the other party gets to tell the judge why this is a bad idea.

The judge will look at all evidence and decide what is best for the child, often this is not what the child wants but what is in the best interest of the child.