34 letters.
From; Mary Poppins.
su⋅per⋅cal⋅i⋅frag⋅i⋅lis⋅tic⋅ex⋅pi⋅al⋅i⋅do⋅cious
[soo-per-kal-uh-fraj-uh-lis-tik-ek-spee-al-i-doh-shuhs]Want to say it backwards? x)
[Cious-do-li-a-pi-ex-tic-lis-gi-fra-li-ca-per-su-suo-ico-dila-ipx-ecit-si-li-gar-fila-crepus]
^ That's impressive. Try it.
The roots of the word have been defined as follows:
super- "above"
cali- "beauty"
fragilistic- "delicate"
expiali- "to atone"
docious- "educable".
With the sum of these parts signifying roughly; "Atoning for educability through delicate beauty."