Translation:The girl's skirt is large and white.
But: Using dative in this form is considered bad manner in German. It's better to use the genitive here: Der Rock des Mädchens ist groß und weiß.
Use of the genitive case is considered a bit stilted nowadays. Most people would use the dative case (as in this example)...
I thought this could also be "The skirt BY the girl is large and white", meaning that the skirt was near the girl, but I got it wrong. Is there something wrong with my sentence or should it be accepted?