in my class two times as many girls as boys.

你是想表达：在你们班级,女孩是男孩的两倍是吧.但是你上面的表达都没有谓语,这应该是个完整的句子才对.
有以下表达：
in my class,girls are two times（or twice) as many as boys.
in my class,there are twice more girls than boys.
倍数表达：
1.times+形容词（副词）比较级+than
eg:This hall is five times bigger than our classroom.
2.times+as+形容词（或much)或副词原级+as...
eg:The big box is four times as heavy as the small one.
3.times+the+名词（size,height,weight,length,width)+of
eg:Your playground is 900 metres wide,three times the size of ours.