That's an interesting question. I dont' know for sure, but the longest one I can think of right now is Electric.
Hope I helped!
Bellybuttons would be the longest word I can think of beginning with a 'b' and ending in an 's'.
firetruck
Acetylcholinesterase is a 20 letter word. It is a chemical compound.
The longest ones I could find in Webster's Second International dictionary are 17 letters long: antiagglutinating autoagglutinating
Scoop, Soup, and others, I'm not 100% sure about the rest though
The longest word starting with E is Electroencephalographically.
chromolithographer (18 letters) counterdemonstrator (19 letters)
Floccinaucinihilipilification is the longest word starting with F; it means the act of estimating something as worthless.
Youthfulness is the longest word starting with the letter Y. It contains twelve letters.
bob
Extraterritoriality is the longest word in the English language beginning with "ex."
Pneumonoultramicroscopicsilicovolcanoconiosis (also happens to be the longest word in the English language)