In earlier versions you could only select IPv4 subnet sizes /24, /16, and /8.
The new version (9.1.1.1) is now available for download from: https://github.com/jhsoftware/sdnspi-SmartReverseDns/releases
For installation instructions and more information about this plug-in, see https://simpledns.plus/kb/187-smart-reverse-dns-plug-in