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
The longest word starting with E is Electroencephalographically.
Scoop, Soup, and others, I'm not 100% sure about the rest though
chromolithographer (18 letters) counterdemonstrator (19 letters)
Floccinaucinihilipilification is the longest word starting with F; it means the act of estimating something as worthless.
bob
Pneumonoultramicroscopicsilicovolcanoconiosis (also happens to be the longest word in the English language)
Extraterritoriality is the longest word in the English language beginning with "ex."
Methionylthreonylthreonylglutaminylalanyl...isoleucine. It has 189819 letters and the longest word in the world.