>>116
% man ppp
...
set mtu [max[imum]] [value]
...

% cat /etc/ppp/ppp.conf
...
set mru max 1448
set mtu max 1448
...

% ifconfig
...
tun0: flags=8051<UP,POINTOPOINT,RUNNING,MULTICAST> mtu 1448
...

Have a fun!