A girl usually refers to an unmarried female while a woman can be either married or unmarried. A single girl can emphasize a girl without a boyfriend as well as an unmarried girl. 单身女孩 means 未婚女孩 or 沒有男友的女孩. Both are okay. The former is formal. But, sometimes, for example, in facesook, the situations divide into more than two (married and unmarried) and single means without a BF/ GF there.

单身女孩

"单身女孩“---“dan shen nv hai”

单身女孩

单身女孩