MTU = Maximum Transmission Unit
The MTU setting controls the maximum Ethernet packet size (in bytes) your ISP (and subsequently your DSL modem) can receive and transmit.
How to determine the MTU size:
ping -s 1472 www.dslreports.com
OS X users:
ping -D -s 1472 www.dslreports.com
Windows 2000/XP users:
Go to Start/ Programs/ Accessories/ Command Prompt and type the following:
ping -f -l 1472 www.dslreports.com
Press Enter. Then reduce 1472 by 10 until you no longer get the "packet needs to be fragmented" error message. Then increase by 1 until you are 1 less away from getting the "packet need to be fragmented" message again.
Add 28 more to this (since you specified ping packet size, not including IP/ICMP header of 28 bytes), and this is your MaxMTU.
Note:If you can ping through with the number at 1472, you are done! Stop right there. Add 28 and your MaxMTU is 1500.
Information gratefully gleaned from here.