how to loop through File stream

You can only read data from the pipe once. If you want to calculate the amount of data, you will need to store it all and count. Something like:

#include <ctype.h>
#include <stddef.h>
#include <stdio.h>
#include <stdlib.h>

static void * xrealloc(void *buf, size_t num, size_t siz, void *end);
struct string{ char *start, *end; size_t cap; };

static void
push(int c, struct string *b)
{
        if( b->start == NULL || b->end >= b->start + b->cap ) {
                b->start = xrealloc(b->start, b->cap += 128,
                        sizeof *b->start, &b->end);
        }
        *b->end++ = c;
}

int
main(int argc, char **argv)
{
        int c;
        int text = 1;
        char *cmd = argc > 1 ? argv[1] : "pwd";
        struct string content = {0};
        FILE *command = popen(cmd,"r");
        if( command == NULL ) {
                perror("pwd");
                return EXIT_FAILURE;
        }
        while( (c = getc(command)) != EOF ){
                push(c, &content);
                if( ! isprint(c) && ! isspace(c)) {
                        text = 0;
                }
        }
        printf("%zu bytes\n", content.end - content.start);
        if( text ) {
                push('\0', &content);
                printf("%s", content.start);
        }
        return 0;
}
static void *
xrealloc(void *buf, size_t num, size_t siz, void *endvp)
{
        void **endp = endvp;
        ptrdiff_t offset = endp && *endp ? *endp - buf : 0;
        buf = realloc(buf, num * siz);
        if( buf == NULL ){
                perror("realloc");
                exit(EXIT_FAILURE);
        }
        if( endp != NULL ){
                *endp = buf + offset;
        }
        return buf;
}