To put it simply, tsunamis can get very big. But I know that is not the type of answer you are looking for.
You might have heard about the 2004 Indian Ocean Tsunami and the 2011 Japanese Tsunami.These two resulted in massive damage to the shore and also resulted in many lives.
The Indian Tsunami started when there was a massive earthquake occurred off the west coast of Indonesia. The earthquake that caused this was more powerful that all the bombs detonated in WWII.The tsunami had the range from the east side of Indonesia to the east side of Africa.This tsunami also made its way 2 kilometers into places as you can clearly see from the picture on the right.This tsunami killed 250,000 people. The most every recorded in history.
The Japanese Tsunami was also created with an earthquake. This tsunami was 40 meters high and went in 10 km.
Unlike regular tsunamis, these can get even bigger. And instead of regular tsunamis, the way they are formed is different. Instead of have an earthquake, there must be a lot of mass going into water at a very fast rate.
The last time this happened was back in 1963 in Italy. The government was building a dam in a valley above a village. The place that they were building it on was not structurally sound. One day, the whole half of the mountain came crashing down into the reservoir and created a 260 meter high mega tsunami that came over the dam and into the villages below. That is almost the same height as the trans-America building in San Francisco!
The last two images above are the village before, and after.
As you can clearly see, mega tsunamis can be very big.