请问各位老师，关于 and 连接两个系表结构的省略问题：

(1)They're usually three meters tall and (they are) as big as the size of a classroom.

(2)They're usually three meters tall and are as big as the size of a classroom.

(3)They're usually three meters tall and as big as the size of a classroom.

以上3个句子都正确

备注：并列连词，引导另外一个分句，如果前面的分句某个成分在后一个分句再次出现，则使用省略结构，即省略前面已经出现过的“主语，谓语，或者是主语+谓语/主语+系动词be”

She is beautiful and (she is 省略) clever。

She is tired ,but( she is省略) happy.

He wanted nothing but(he wanted 省略) to stay here.

除了留在这里之外,他别无所求。

It is not water, but(it is省略) alcohol.

这不是水，而是酒精。

解答如下：

主语相同的情况下，主语必须要省略，后面系动词are可以省略，也可以不省略。都正确。