the size在此意为：how large or small a person or thing is

an area the size of (= the same size as) Wales

I saw a man the size of Yao Ming. 我看到一个和姚明一样高的人。

I saw a spider the size of (=the same size as) my hand in the backyard.

名词+the size of...为名词作后置修饰语。实为省略了介词of, 等于名词+of the size of...

例如BNC例句：

 it acts only over distances of the size of a nucleus, about 10-13cm.

relatively small regions of the size of our galaxy will be found here and there

所以，网友句子中的a chunk a quarter of the size of Wales

= a chunk of a quarter of the size of Wales

= a chunk of the same size as a quarter of Wales

相当于威尔士四分之一面积的一大块