http://www.experts-exchange.com/Hardware/Networking_Hardware/Firewalls/A_3110-Setting-WAN-MTU-Size-For-Sonicwall-Appliances.html
Run
ping www.google.com -f -l 1500
and reduce 1500 in increments of 100 until it stops saying packets can't be fragmented
then increase again until you find the max number that doesn't give this error
then add 28
