How to find size of a character array from its pointer in C

Question

How should I define the function size_of so that it will return me 25, size of array A from pointer p.

#include <stdio.h>
#include <string.h>

int main()
{
    char A[25], B[]="My String";
    strcpy(A, B);
    size_of(A);
    return 0;
}

int size_of(char *p)
{


}