Something totally stupid..
Hexakosioihexekontahexaphobie
It's a word that means "phobia of the number 666", it contains 29 letters, and could be one of the longest words in French (medicine domain though), and that's not counting the real name of some complex proteins, that can contain several hundred letters, or something like this :
chlorure d'aminométhylpyrimidinylhydroxyéthylméthythiazolium, which is the name of the riboflavine (B vitamin)..
I should "really" go to bed, I'm getting beyond silly !