Changing MTU Size in OS X

To change the MTU size temporarily (for testing purposes)

1. Go to /Applications/Utilities/ and launch Terminal
2. Type sudo -s and press Return
3. Enter your administrator password when prompted and press Return
4. Type ifconfig ppp0 mtu 1430 (this figure can be substituted by any MTU size) and press Return
5. To check that the change has gone through, type ifconfig ppp0 ¦ grep mtu and press Return

NOTE: these settings will be lost when the connection is reinitialised (eg. through a reboot).  An alternative method to keep these settings permanent is provided in this Apple support document.