I think its all subjective . Whatever a girl is successful with is what they should be swinging . At our last batting practice I recommended that the girls try out as many different bats as they could lay their hands on to see how they felt and what they were comfortable with . Several girls wound up changing what they liked and almost all of them saw their hitting improve . Its pretty tough to get a feel for what they will be successful with standing in a sporting goods store or reading online . I know that my DD's journey involved about a dozen bats and untold hours in a batting cage before she finally found a bat she liked .
All that being said , if the girls can swing a big bat successfully then go for it .