Obviously i am late. But it may help someone.
#ip link set ppp0 mtu 1452
add it to start up script somewhere
ppp0 - is ppp network interface inside linux kernel. where 1452 - mtu size actually.
To see if you have ppp0 interface do "ip link"
I assume that iproute is installed.
http://www.policyrouting.org/iproute2.doc.html