-----Description----- 
This task requires writing a Lean 4 method that calculates the surface area of a cube based on the length of one of its edges. The method should compute the surface area using the standard formula for a cube.

-----Input-----
The input consists of:
size: An natural number representing the length of an edge of the cube.

-----Output-----
The output is an natural number:
Returns the surface area of the cube.
