I have committed an implementation in http://svn.apache.org/r1857624 , using Evgeny's new libsvn_fs ioctl API.
It is feature-complete (old and new FSFS formats, revs and revprops, packed and unpacked. The CLI is: $ svnadmin rev-size /path/to/repo -r1 1337 bytes in revision 1 plus a 'quiet' option. There are a couple of opportunities for optimization marked in the code: 1. for the size of an unpacked rev-file, we unnecessarily read the indexes 2. for the size of packed revprops, we unnecessarily read the props -- - Julian