why do you have to specify dimensions of array in c code example

Example 1: size of an array c

int a[20];
int length;
length = sizeof(a) / sizeof(int);

Example 2: increase size of array in c

#include <stdio.h>#include <stdlib.h>int main(){//a pointer to dynamically allocated memory from the heap is returned.	int *a = (int *) malloc(20 * sizeof(int));for(int i = 0; i < 20; i++)a[i] = i + 1;printf("\nThe contents of the array are: \n");for(int i = 0; i < 20; i++)printf("%d\t", a[i]);realloc(a, 40);for(int