These pieces of wood are some of them far larger than any trees that you have ever seen;

划线部分为“形容词短语”，作后置定语。这个“形容词短语”，只不过是个比较级而已。在trees 后面又跟了一个定语从句。

这是英语很常见的现象，读句子要连贯，不可拆分意群。等于：

These pieces of wood are some of them (which are) far larger than any trees that you have ever seen;

“形容词短语”，作后置定语。举例：

1. an actor suitable for the part 适合扮演这个角色的演员。

2. a manager responsible for the work 负责这项工作的经理

3. a basket full of vegetables 一只装满蔬菜的篮子

4. people present at the meeting 出席会议者

5. a house larger than mine 比我的房子更大的一幢房子