I guess the longest fictional word I know is: <em>supercalifragilistichespiralidoso</em>.

The longest real word I know as a native, is:

<em style="color: rgb(100, 105, 103);">psiconeuroendocrinoimmunologia. </em>

Do you know any others which are very long? And what about the longest in your native language(s)? <em style="color: rgb(100, 105, 103);"> </em>