-fix calculation of block size, doxygen

git-svn-id: https://gnunet.org/svn/gnunet@31664 140774ce-b5e7-0310-ab8b-a85725594a96
-fix #3233

git-svn-id: https://gnunet.org/svn/gnunet@31665 140774ce-b5e7-0310-ab8b-a85725594a96