With the Libero position, height is not a determining factor - the ability to pass and play defense is. If the talent level is the equal, some college coaches prefer a shorter libero because they can be quicker, while other college coaches prefer a taller libero because they have a longer reach/wingspan.
I would STRONGLY encourage you to encourage your daughter to play the Outside hitting position, especially if she does hit that 5'10-6' height. She is still young and developing her attacking power, so give it time - An OH that can pass, attack, block, play defense and serve, who is also 6' tall will have more collegiate options than a 6' Libero.
Now, that being said, if she is happiest just playing Libero, and wants to focus on that position, then keep her there and promote her abilities to the appropriate level of colleges.
Athletes will be more successful when they are playing/competing in their specific comfort zone - If an athlete is playing in a position they don't wish to play, they will not be as successful and won't have an enjoyable experience.
Talk with your VolleyPSA and try to determine which position she feels the most comfortable playing or wants to play, and then move forward.