The main building boasts floor area of over 18,000 square metres, of which exhibition area is 8,300 square meters.

該句中of which前面沒名詞，直接跟在主句之后，但exhibition area確也是主語的一部分。

請問各位老師，這種使用方法是不是成立呢？

of 的功能之一就是表示部分與整體的關系：A of B 中，B 是整體，A 是部分。

你可以通過以下例句領會一下 A of B 的使用方法：

Of the 25 candidates who ran for the Democratic presidential nomination this year, 18 supported statehood.

還原：18 of the 25 candidates who ran for the Democratic presidential nomination this year supported statehood.

再看下你的例句：

The main building boasts floor area of over 18,000 square metres, of which exhibition area is 8,300 square meters.

A 是 8,300 square meters，B 是 over 18,000 square metres（即關系代詞 which 代替的先行詞）。

這個例句等于 The main building boasts floor area of over 18,000 square metres, of which 8,300 square meters is exhibition area.

或者 The main building boasts floor area of over 18,000 square metres, 8,300 square meters of which is exhibition area.

你的例句中，是定語從句中主語（8,300 square meters）和表語（exhibition area）倒裝現象。