You can’t modify a statically allocated variable array.

You can however use the memory allocation functions to create and modify the array:

#include <stdio.h>

// global 

int size = 0;
char *array = NULL;

int main(){
    scanf("%d", &size);
    array = malloc(size);
    if (!array)
    {
        ... handle error.
    }
    scanf("%d", &size);    //ask a new dimension
    char *tmp = realloc(size);    /reallocate array keeping old data
    if (!tmp)    //reallocation failed
    {
        free(array);    //release memory
        ... handle error.
    }
    array = tmp;     //reassign new memory to the array pointer
}