Biggest word?

No, it’s not that word. And that isn’t what “irony” means, either.

A fear of big words is: Hippopotomonstrosesquipedaliophobia, at 35 letters.

It depends on what you mean by a “word.” If you count technical words that aren’t actually in the dictionary, spelling out the chemical formula for the protein titin would give you a word of over 180,000 letters. But most people do not accept technical terms as words.

The longest word found in most major dictionaries is: Pneumonoultramicroscopicsilicovolcanoconiosis, at 45 letters.