The name of the entry is 'MTU'. You might want to compare your results with
this manual setting and using a REG_DWORD entry called PMTUDiscovery with a
value of 1.
Go to www.dslreports.com/tools and run the Tweak test, it will tell you if
your MTU needs changing. If you want, or need, to change the MTU there's a
utility called DrTCP on that page that you can use. It saves you digging
around in the registry.