Actually, it's the fear of extremely long words, and its validity as a word is questionable. Sesquipedaliaphobia is the fear of long words.
floccinaucinihilipilification, to my knowledge, is the longest word in the english language to qualify. It means 'the act of estimating something as worthless'. Pneumonoultramicroscopicsilicavolcanoconeosis doesn't qualify, as it's a medical portmanteau. It's a lung condition caused by the inhalation of fine silica dust.