What is the proper MTU settings when using free5GC?

Hello,

Let me ask about proper MTU settings when using free5GC.
In the Installation document, it suggest us to set MSS to 1400.
So I thought MTU would be 1440.

sudo iptables -A FORWARD -p tcp -m tcp --tcp-flags SYN,RST SYN -j TCPMSS --set-mss 1400

However, when I tried UERANSIM, MTU was set to 1400 in the source codes.

Does anyone know what would be the appropriate MTU?
I appriciate for your help.