Your example is flawed because "big" is limitless in English. While in many cases people might use "big" as a standard word and more fancy words like "gigantic" to describe something that's extremely big, ultimately big and gigantic are synonomous. Montana is called the Big Sky Country, and the sky is pretty freaking big. The tallest mountain in the world is a big mountain. The Nile is a big river. There's nothing inaccurate or needing improvement about calling a Tsunami a big wave. The reason only a few people understood you is that you were trying to express a sentiment that was inaccurate in relation to the word "big". You wanted to say that the word "big" was not enough and another word should be used instead but there is no other word that means anything bigger than "big".
When people want to convey the sentiment you seem to be reaching for, the most common expression that is logical for the language is "there are no words to describe it" or "there are no words" or just "no words".
You tried to say that "big" was inadequate and another word should be used instead, but few people understood you because logically there is no other word that means something bigger. If, instead, you had used the "there are no words" expression meaning big is inadequate and there is no word in English that properly describes the enormity of the Tsunami, that makes more sense and everyone should understand you clearly.