Based on a question in #Macintosh in irc.quakenet.org here the short solution for the MTU question in OSX Tiger.

In this small example we use my internal ethernet device:

1. From the Apple Dock, choose System Preferences.
2. Select the Network pane.
3. From the Show pop-up menu, choose Built-in Ethernet.
4. Click the Ethernet tab.
5. From the Configure pop-up menu, choose Manually (Advanced).
6. Select the radio button for Custom.
7. In the field to the right of Custom, enter your new setting. Some ISPs may have a specific suggested MTU value. If you’re changing it without a specific recommendation, make incremental changes such as 1500 to 1400 to 1300, and so forth.

Apple offers a much more detailed documentation on this, so continue reading there.

Tags: 10.4, change mtu, howto, MTU, network