That's an implementation defined limit in practice. rsyslog for example defaults...

viraptor on May 28, 2014 | parent | context | favorite | on: Nginx 1.7.1 adds syslog support

That's an implementation defined limit in practice. rsyslog for example defaults to 2K (apparently to be compatible with upcoming RFCs), but it's configurable to higher values. If you want to be compatible with other software though, it may need to stay at 1K.