Size of nifi flowfile in Groovy script

Hello. I am trying to determine a way to get the size of a NiFi flowfile
within a Groovy script. Has anyone done this before, and can tell me how to
do this from within Groovy?

This
https://jameswing.net/nifi/nifi-internal-fields.html
describes fileSize as a hidden field that can be accessed via expression
language, but I do not see how I can reference that in a Groovy script.

Thanks in advance for any help.