What word has the most letters?

It depends who you ask.

PneumonoultramicroscopicsilicovolcanoconiOSis, which is a lung disease, has 45 letters.

There is also a Word which is 189,819 letters long. The Word starts with "Methionylthreonylthreonylglutaminylarginyl... ...", and the full Word is even too long to be typed here. However many people dispute whether it is actually a real Word. Read more about that here (click).