Pimcore\Model\Asset::getFileSystemPath PHP Method

    public function getFileSystemPath()
    {
        return PIMCORE_ASSET_DIRECTORY . $this->getRealFullPath();
    }

 /**
  * Get size of file in bytes
  *
  * @return integer
  */
 function getSize()
 {
     return filesize($this->asset->getFileSystemPath());
 }