Get file size from ACF repeater field

As far as I know, WordPress has nothing built in for this, I would just do. Same thing did you also. So make sure that the $attachment_id having value.

You can use custom function by place that function code in your function.php file.

function getSize($file){
    $bytes = filesize($file);
    $s = array('b', 'Kb', 'Mb', 'Gb');
    $e = floor(log($bytes)/log(1024));
    return sprintf('%.2f '.$s[$e], ($bytes/pow(1024, floor($e))));
}

After place above function code, you can use getSize($url); where ever you use into your template. Here $url is reference of your file path.

Let me know if this stuff not help you.