Question

He is of medium build/height 这句话中的be of 是什么意思？

Answer 1

由于medium height是名词，而is后面接表语，因而在名词前加be of相当于形容词。
of不能去掉
意思与“He is medium high”相同。

Answer 2

is of +名词的形式其实表达的就是形容词的意思

这句话的意思是：他中等个头。=He is medium high.

这个用法还可以表示为：
1.Hope that this information could be any of help。希望这些信息对您有点帮助。＝Hope that his information is helpful.

2. This issus is of importance.这是个很重要的问题。＝This issue is important.

Answer 3

be of +n 相当于be +adj.

Answer 4

be+形容词=be of + 名词
he is helpful.
he is of help.