Hi, 2 ways...
Sys Prefs>Network, highlight the Interface you use for Internet, Adavanced>Hardware tab>Configure Manulaly>MTU.
You can temporarily test by pasting this into Terminal, (will go back to defaults when you reboot)...
sudo ifconfig en1 mtu 1000
Or en0 for ethernet & whatever value you want.
Different Servers/Routers/Modems may have different MTUs, and that can cause Havoc, here's some guides on that issue...
Setting MTU size is a process of trial-and-error: start with the maximum value of 1500, then reduce the size until the problem goes away. Using one of these values is likely to solve problems caused by MTU size:
• 1500. The largest Ethernet packet size; it is also the default value. This is the typical setting for non-PPPoE, non-VPN connections. The default value for NETGEAR routers, adapters and switches.
• 1492. The size PPPoE prefers.
• 1472. Maximum size to use for pinging. (Bigger packets are fragmented.)
• 1468. The size DHCP prefers.
• 1460. Usable by AOL if you don't have large email attachments, etc.
• 1430. The size VPN and PPTP prefer.
• 1400. Maximum size for AOL DSL.
• 576. Typical value to connect to dial-up ISPs