Force FileVault to use 256-bit key encryption
Most documentation I'm finding seems to indicate that the key's used in macOS 10.9 and higher use a 256 bit key by default.


  Encryption and authentication support
  
  FileVault 2 uses the Advanced
  Encryption Standard (AES) encryption algorithm, which delivers robust
  protection for stored data. Until mid-2013, it only supported the use
  of 128-bit keys, not 256-bit keys. Although 128-bit keys are
  technically acceptable in many environments, organizations are rapidly
  moving toward 256-bit keys to thwart emerging threats.
  
  The latest versions of Mac OS X, starting with 10.9, support 256-bit
  AES keys, so organizations wishing to enable FileVault 2 on legacy
  systems should be cautious about the 128-bit key strength present in
  older Mac OS X versions, Lion (10.7) and Mountain Lion (10.8).


Reference: Apple FileVault 2: Full disk encryption software overview

This PDF and the Wikipedia page seem to indicate this as well.


macOS Security - Overview for IT 2018
Wikipedia - FileVault