Built in cleanup procedures are documented here: https://help.sonatype.com/display/NXRM3/Cleanup+Policies
If your instance is old, recommend upgrading to get access to everything on that page for sure.
Likely what you want however is https://issues.sonatype.org/browse/NEXUS-10821. Until that is implemented you are somewhat limited to a date based approach. I am skeptical there’s a scripted solution (and not one I’ve heard of) but I could be wrong.
Hope that helps a little,
Joe