The blast was the equivalent of 200 megatons of TNT. For perspective, the largest explosion ever made by humans was the detonation of a Russian hydrogen bomb, which was 50 megatons. That blast broke windows in buildings 560 miles away. Krakatoa was four times that; the cloud it generated wiped entire villages off the map 25 miles away and created a tsunami that traveled all the way to South Africa. That wasn't all Krakatoa's neighbors got for their birthday that year; giant pieces of rock and coral reef fell from the sky as well.
Now get this: Krakatoa is only number five on the list. Link