It's not normally necessary to set it. It should negotiated during the connection process.
For example my WAN here at home is PPPoE, I have not set any MTU or MSS value at the interface setup. Also in the UK.
In the PPP log I can see:
Jan 6 18:09:01 ppp: [wan_link0] MRU 1492
Also if I interegate the interface at the command line I can see:
[2.1.5-RELEASE][root@pfsense.fire.box]/root(2): ifconfig pppoe0
pppoe0: flags=88d1 <up,pointopoint,running,noarp,simplex,multicast>metric 0 mtu 1492
inet6 fe80::290:7fff:fe3c:9609%pppoe0 prefixlen 64 scopeid 0xd
inet 87.113.*.* --> 195.166.*.* netmask 0xffffffff
nd6 options=3 <performnud,accept_rtadv></performnud,accept_rtadv></up,pointopoint,running,noarp,simplex,multicast>
Steve