human readable file size

function humanFileSize($size)
{
    if ($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;
}

... works great except: I can't manually choose in what format I need to display, say i want to show in MB only whatever the file size is. Currently if its in the GB range, it would only show in GB.

Also, how do I limit the decimal to 2?