How to find the proper MTU size for Network

 Summary

One of the easy and most accurate ways to test for optimum MTU is to do a simple DOS Ping test. You will simply send out ping requests and progressively lower your packet size until the packet no longer needs to be fragmented. Although this simple test is accurate for testing end points, users may find that a lower MTU may be better for their particular circumstances. 
Important Note: MTU must be 1492 (or lower) when using PPPoE connectivity.

MTU (Maximum Transfer Unit)

Network                          MTU (bytes)
-------------------------------------------------
16 Mbps Token Ring      17914
4 Mbps Token Ring          4464
FDDI                                4352
Ethernet                            1500
IEEE 802.3/802.2             1492
PPPoE (WAN Miniport)   1480
X.25                                    576

Proper Value 1404

How do I find the proper MTU size on my router?

To find the proper MTU Size, you'll have to do a special ping of the destination you're trying to go to. A destination could be another computer, or a URL.

Open Teminal or Command Prompt

Core Basic Command
ping [url] [-f] [-l] [MTU value]

Example: ping yahoo.com -f -l 1472
Example: ping www.tp-link.com -f -l 1472
ping www.dslreports.com -f -l xxxx
ping www.google.com -f -l xxxx
ping www.dslreports.com -f -l 1472

in Ubuntu
Sudo ping yahoo.com -f -l 1472

Drop the test packet size down (10 or 12 bytes) and test again. Notice that the packet still needs to be fragmented

Drop the test packet size down more and test again until your reach a packet size that does not fragment

Once you have a test packet that is not fragmented increase your packet size in small increments and retest until you find the largest possible packet that doesn't fragment.

Take the maximum packet size from the ping test and add 28. You add 28 bytes because 20 bytes are reserved for the IP header and 8 bytes must be allocated for the ICMP Echo Request header. Remember: You must add 28 to your results from the ping test!

An example:
1440 Max packet size from Ping Test
+ 28 IP and ICMP headers
1468 is your optimum MTU Setting

                                              Don't forget to