Translation:The girl's shoes are too small for her.
Both the singular and plural are "sko". As Xneb pointed out, it is only the word "små" that tells you that "sko" in this sentence refers to shoes.
If you wanted to say the "The girl's shoes are too small for her" in Danish you would have to say "Pigens sko er for lille til hende."
It's a bit like the difference between "the sheep is lost" and "the sheep are lost"