* Most dictionaries: The longest word is "pneumonoultramicroscopicsilicovolcanoconiosis" with 45 letters. It refers to a lung disease caused by inhaling very fine silica dust.
* Some dictionaries: They include "floccinaucinihilipilification" (29 letters), which means "the act of estimating something as worthless".
* Specialized dictionaries: Some dictionaries include even longer words, often constructed specifically to be long. For example, "antidisestablishmentarianism" (28 letters) and "honorificabilitudinitatibus" (27 letters) are sometimes found, though they are not commonly used.
So, the answer depends on your definition of "dictionary"!