He is of medium height.中为什么加of?

be + of + 名词 相当于 be + 这个名词对应的形容词
比如
to be of use = to be useful
to be of help = to be helpful
to be of importance = to be important
to be of interest = to be interesting
to be of assistance = to be helpful (没有"assistanceful")
to be of worth = to be valuable (没有 "worthful")
to be of good quality = to be good with respect to quality (没有"good-quality-ful") 等等等等
你可以发现很多时候我们需要用名词来修饰限定,它却没有相应的形容词.汉语里你可以直接加一个“的”.比如这里“中等身材”,中文直接变成“中等身材的”.英语构词没这么方便,那么用of + 名词 可以达到相应的效果.
不仅如此,用了 of + 名词 的形式后,你还可以用更多的形容词来修饰这个名词,来达到更好的说明效果.比如 of great help.避免了greatly helpful.尤其口语中,短而有力的词效果更好.
至于跟she is a girl完全不一样.be of +名词 等同于用对应的形容词作表语,而不是那个名词.