Chapter 1 What’s New for FortiOS 5.2.1 : IPv6 : TCP MSS Values
  
TCP MSS Values
TCP MSS values for both the sender and the receiver can now be set for IPv6 policies using the CLI.
Syntax
config firewall policy6
edit <index_int>
set tcp-mss-sender <value>
set tcp-mss-receiver <value>
end
end