The longest word starting with B is:
brachopsedo pnemonoultra microscopics ilicovolcano coniosis
(the word has no spaces but the whole word will not appear unless it is broken into pieces).
Bellybuttons would be the longest word I can think of beginning with a 'b' and ending in an 's'.
Including the letters d, b, f, the longest word might be defibrillation.
21-letter wordbuckminsterfullerenes
pub
Biotechnological
The longest one that -I- know would be ballistocardiograph which is an instrument for detecting body movements caused by heartbeat.
DETECTED
Decade
you are rubbish arent you
14-letter wordsbourgeoisified, bureaucratised, bureaucratized, butterfingered
blower, bowler, nobler
1,909 letters in the longest word