The main building boasts floor area of over 18,000 square metres, of which exhibition area is 8,300 square meters.
该句中of which前面没名词，直接跟在主句之后，但exhibition area确也是主语的一部分。
请问各位老师，这种使用方法是不是成立呢？
of 的功能之一就是表示部分与整体的关系：A of B 中，B 是整体，A 是部分。
你可以通过以下例句领会一下 A of B 的使用方法：
Of the 25 candidates who ran for the Democratic presidential nomination this year, 18 supported statehood.
还原：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（即关系代词 which 代替的先行词）。
这个例句等于 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，是定语从句中主语（8,300 square meters）和表语（exhibition area）倒装现象。