There is much to be said about optimizing the Maximum Transmission Unit (MTU) size on your Meraki MX. The MTU is the maximum size of the packet transmitted over the LAN / WAN.
Too big an MTU means the packets sent are fragmented when they hit a router that only supports an MTU smaller than that which is set by default or otherwise. The default MTU of a Meraki MX is 1500 bytes (seen as a safe setting.)
You can test whether a smaller MTU size would be advantageous by following the article below:
When I test our MX with the value in the terminal line set to 1472 (MTU 1500 - which is 1472 +28 for the headers), it fragments. Then when I test it with a value of 1464 it returns correctly with no fragmentation. (MTU 1492 - which is 1464 + 28 for the headers).
If you do the troubleshooting as per the article and find you need to take action to reduce the size of the MTU, then there are a few ways to do this. You might want to check with your ISP what their recommended MTU settings are first.
Here are some steps you can take when dealing with an MTU issue.