In the Scrabble dictionary, there are twenty-five fifteen-letter adjectives that I could find, but none with more letters than that. In Webster's Second International, I could find six twenty-letter adjectives, but none over that. There may be others that don't take the usual adjectival forms.
Fifteen-letter adjectives from the Scrabble dictionary:
ecclesiological
echinodermatous
electroacoustic
electrochemical
electrodialytic
electromagnetic
electronegative
electrophoretic
electropositive
electrosurgical
enantiomorphous
encephalopathic
endonucleolytic
enterobacterial
entrepreneurial
epidemiological
epistemological
epitheliomatous
ethnohistorical
excommunicative
excrementitious
exhibitionistic
exoerythrocytic
expressionistic
extralinguistic
Twenty-letter adjectives from Webster's Second International:
electrocardiographic
electrochronographic
electrometallurgical
electrophysiological
electrotherapeutical
epididymodeferential
Erectile
The longest word that starts with q is quintoplets!!!What about Quartermasterships?
The longest word starting with E is Electroencephalographically.
methionylglutaminylarginyltyrosylglutamylseryllleucylpheny
supercalifragilisticexpialidocious.
Kennethrobertofitzwaterayjameseepatriciacashiichelsayybrianaeasthamaiecaittlinapaigerboothean.
Lymphadenopathy
A word that starts with E and ends with A: extravaganza.
SMILER - There is a mile between the s and the r!
A good word for a cheerleader that starts with E is energetic.
An appropriate word for horse that starts with an E is "equine."
A word that starts with E and ends with U: ecru.