anyone familiar with setting mtu size?

I changed my router and started having issues using rdp when I would vpn into our network. I found that mtu size may have been the culprit. I found the mtu size test and ran it and ended up at like 1464. well, now I start having issues loading pictures and stuff on forums. I run the mtu dos test again and it comes back with a lower number. I set my router and pc to the new lower number and run the test again and it comes back with an even lower number.

doesnt make much sense to me.