Well you can use SIZE = atoi( argv );
where atoi() is declared in <cstdlib>
However, you shouldn't set the size of a standard array with anything that isn't known at compile time (though some compilers permit it). You should use either a dynamic array, created with new, or let the compiler do the memory management and use a vector<int>.
int main(int argc, char *argv)
if (argc < 2)
cout << "Missing size parameter\n";
int size = atoi(argv);
if (size <= 0)
cout << "size " << argv << " must be greater than zero\n";
// C++ recommended way - use vector
// Possible alternative, but not recommended - use new and delete 
int * array2 = newint[size];
Code which uses the array
// Before exit, de-allocate array2
delete  array2;
is not possible in standard C++, because len is not a compile-time constant. It may be allowed by your compiler, but could fail if a different compiler is used. It is best not to rely on such quirks, especially when learning. Use a vector, or maybe new  and delete.