The biggest word could be interpreted as the most important or the most influential. That could be God or Stop! For instance. However, I think you mean the longest word. In the English language there are several contenders. You might expect the medical profession to come up with a few and pneumonoultramicroscopicsilicovolcanoconiosis - a kind of lung disease caused by inhaling quarz - is certainly a whopper.
But some would say that disease names don't count because they're not in everyday speech. So that rules out the very longest word of all, the chemical name for the titin protein found in humans. Its full name has 189,819 letters.
I'd go for this word: Floccinaucinihilipilification. It means when you regard something as valueless. No, it's not in everyday speech but that's not the fault of the word. It's the fault of those who know it and don't use it!
