Hi Paul,
It's been a while since I've had a play with this but I think you just add the settings to /etc/ppp/options or /etc/ppp/options.pptpd
To template the changes:
mkdir -p /etc/e-smith/templates-custom/etc/ppp/options.pptpd
mcedit /etc/e-smith/templates-custom/etc/ppp/options.pptpd/mtu
Add whatever value you want here
mcedit /etc/e-smith/templates-custom/etc/ppp/options.pptpd/mru
Add whatever value you want.
/sbin/e-smith/expand-template /etc/ppp/options.pptpd
service pptpd restart
The mtu and mru are usually negotiated by the client, but by adding these settings to the server it should override the client.
Regards Lloyd