Use a standard A4 paper to form a largest volume block

how to Use a standard A4 paper to form a largest volume block ?

You mean the block X so that X has an A4 paper as its surface, and has the maximum volume?

The area of a sheet of A4 paper is 2^-4 m² = 0.0625 m².
To get the maximum volume you use a cube (a sphere would be better, but that is not a 'block').
The six sides will each have area A=0.0625/6 m².
The sides are the square roots of that, s=sqrt(A)≈0.0104.
The volume is s³≈0.00106 m³.

Of course, whether you can actually fold a sheet of paper this way is a different matter, but by (possibly infinite) cutting and pasting it is possible.

From working in typesetting, I can assure you that an A4 sheet of paper is precisely 0.21 m by 0.297 m = 0.06237 m². Not that it makes a huge difference here...

Weren't those cube templates always something similar to this:

(Assuming you had to cut it out, and weren't allowed to paste)

Code:

---

  _
  |  |
  _
  |  |
  _
| |  |  |
  |  |
  _

---

forgive the crude drawing..
lol

basically a line of four squares, with two more equal sized squares coming off the line..