Anthony

The easy way to do that is to ping from one PC to the other, varying the size of the ping packet and specifying do not fragment. In the Windows ping use the -l parameter to control the packet size and use -f to indicate do not fragment. I would start at 1500 with no fragment and see if it works. If so then your question is solved. If it does not work I would start sending pings with smaller sizes till you find the one that starts to work.

HTH

Rick