ping www.google.com -f -l 1492
1440 Max packet size from Ping Test
+ 28 IP and ICMP headers
1468 is your optimum MTU Setting
Normally, MSS is equal to MTU minus 40
References
https://kb.netgear.com/19863/Ping-Test-to-determine-Optimal-MTU-Size-on-Router
http://www.tp-link.com/us/FAQ-190.html
https://forum.peplink.com/t/how-to-determine-the-optimal-mtu-and-mss-size/7895