PHP

<?php
function file_size($url){
    $size = filesize($url);
    if($size >= 1073741824){
        $fileSize = round($size/1024/1024/1024,1) . 'GB';
    }elseif($size >= 1048576){
        $fileSize = round($size/1024/1024,1) . 'MB';
    }elseif($size >= 1024){
        $fileSize = round($size/1024,1) . 'KB';
    }else{
        $fileSize = $size . ' bytes';
    }
    return $fileSize;
}
echo file_size('0.rar');
?>