1)系动词are后面的of sufficient size为什么要有of？of在此表达什么含义？

答: of sufficient size 是英语中的常用搭配，其中的 of 表示特征或特点。类似的如：

When I was of your age, I was in the country.

Here is a piece of wood of the right size.

He has an album of the same thickness as mine.

The handkerchief is of a very smart design.

The two boys are of exactly the same weight.

2)是否可以去掉of，或者直接变为but only 284 and 72 of those are sufficient for our grafits.

答: 完全可以变为but only 284 and 72 of those are sufficient for our grafits.

3)关于for our grafits中，for是什么含义？我还有点分不清如何判断for our grafits修饰的是系表结构还是size的定语。

答: 这里用 for，是与 sufficient 构成搭配。for our grafits是状语，表示对象。