Is it correct to say "von dem" instead of "vom"?
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.
I think you meant genitive.
Of course I did. Sorry.
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?
It means: "The skirt of the girl is...." It belongs to the girl or at least is being worn by the girl. There is no place involved. It is possessive.