My wife and oldest daughter are both 5' 8". Wife has longer legs, and daughter has longer torso (like Dad). They both fit (more or less) on a traditional 53-54 cm frame. My wife rides a 53cm Trek with a shorter, more upright stem, and my daughter rides a 52 cm Bianchi Volpe that has a somewhat long top tube. She has a longer stem.

Long way of saying...it depends. Your girl might do well on a 52-53 cm modern frame with some fiddling with the stem reach. You could take her measurements and use a fit calculator to get a sense of the top tube that you need to begin.