Use the Ping command with the syntax:
C:\ ping www.google.com -f -l 1500
If you get a “Packet needs to be fragmented but DF set” response, lower the packet size in 8, 10 or 12 bytes.
Continue lowering the packet size until you get a successful reply (packet size not fragmented).
Then increase your packet in 2 bytes intervals until you get the largest not fragmented packet. In my case 1472 bytes.
Add 28 bytes to this number and that will be optimum MTU setting. In my case: 1472 + 28 = 1500 bytes.