It(主语）// is（系动词）// now an entertainment colossus（表语）// the size of which the world has never seen（修饰表语后置定语：定语从句从当）.

参考译文：它现在是一个巨大的娱乐场所，其规模是世界以前从来没有出现过的。

备注：关系代词which代替先行词“an entertainment colossus”并在定语从句作介词“of”宾语或者说属于“介词+关系代词”引导定语从句

entertainment colossus the size of which the world has never seen

=an entertainment colossus of which the size the world has never seen

上面的定语从句可以转换为下面几种定语从句结构

It is now an entertainment colossus the size of which the world has never seen.(the size of which作定语从句谓语has seen的宾语）

=(1)It is now an entertainment colossus of which the size the world has never seen.(of which the size作定语从句谓语has seen的宾语）

=(2)It is now an entertainment colossus whose size the world has never seen.

(whose size作定语从句谓语has seen的宾语）