答案为 D。so可以修饰形容词，如：

It is so big.

He is so young.

The book is so expensive.

但是，如果 so 修饰的形容词后面带有被形容词修饰的名词，则要注意：这个名词只能是单数可数名词，且必须连用不定冠词，且这个不定冠词总是位于“so+形容词”之后，名词之前。如：

It is so big a box.

He is so young a student.

It is so expensive a book.

siza应为size，so small a size = such a small size。

当形容词修饰表泛指的单数可数名词时，不定冠词“a (an)”通常位于形容词之前，但当形容词被how, so, as, too, that, enough等程度副词修饰时，要将副词和形容词一起置于冠词前面。例如：

He is so good a boy that we like him. 他是个这么好的男孩,所以我们都喜欢他.

That’s too hard a task for me. 那对我来说是太艰巨的任务。

She is careful enough a nurse to tend patients. 她是一位照料病人足够细心的护士。

▲ rather和quite也是程度副词，但其强调程度远不及how, so, as, too, that, enough。所以，它们既可以放在不定冠词前也可以放在不定冠词后，但不能将其与形容词一块提到不定冠词前。例如：

He came across to me as being quite a / a quite nice person. 他在我的印象中是一个挺好的人。（不能说quite nice a person）

Forrest was rather a / a rather stupid boy. 佛里斯特是个相当愚蠢的男孩。（不能说rather stupid a boy）

It would not prove difficult in a village of so small a size.

A. a such small  

B. so small

C. the smallest

D. so small a

答案解析如下:

D为正确答案，属于so+adj.+a+名词结构

关于固定冠词a与限定词的位置规律如下所示：

不定冠词用于固定结构quite/half/rather/many/what/such a (an)+名词或so/too/how+形容词+a(an)+名词中，表示程度、数量或感叹等.

A:quite/half/rather/many/what/such a (an)+名词

(1)half a+时间名词

It takes half an hour to get there. 

The visitor left half an hour ago.

(2)quite a+adj.+名词

He's quite a famous artist. 

I think Chinese is quite a useful language.

(3)many a+adj.+名词

Many a man would welcome such an opportunity. 

Many a student has asked that question.

(4)what a+adj.+名词

What a fool he is!  

(5)too+adj.+a+名词

It's too difficult a problem for him.

(6)rather +a+名词

Tom is rather a fool.
(7)what+ a+adj.+名词

What a fine day it is!

(8)such +a+adj.+名词

How can you say such a thing?

B:so/too/how+形容词+a(an)+名词

(1)so+adj.+a+名词

We have not had so hot a day before.

(2)as++ adj+a+名词
Susan is as clever a girl as Anna.

(3)as++ adj+a+名词

That is too difficult a book for beginners.

(4)how+ adj+a+名词

No one knew how serious a problem it was until later.