A ping (deterining MTU) question

I want to know my optimum MTU so I connected to the internet, went to MS DOS, and typed >ping -f -l {my trial MTU} www.{my ISP} and clicked enter. From a trial number of 549 and above, I got a message "Packet needs to be fragmented but DF set",so I have to try a lower value. The last number possible which is 548 gave me a message:
Request timed out
Hardware error
Hardware error
Hardware error
Packets sent 4 Received 0 Lost 4 (100% loss)
Approximate round trip time Min 0 Max 0 Ave 0
Anybody knows why is this?

Well? I also got my answer. Its the firewall that blocked my echo request. Just for those who want to know.