The longest words in Oxford Dictionaries Online are:
antidisestablishmentarianism - opposition to the disestablishment of the Church of England - 28 letters
floccinaucinihilipilification - the estimation of something as worthless - 29 letters
pneumonoultramicroscopicsilicovolcanoconiosis - a supposed lung disease - 45 letters
You're unlikely to come across these words in genuine use: they're generally just provided as answers to questions about the longest words in the English language. In terms of sheer size, however, the longest word to be found in Britain is the Welsh place nameLlanfairpwllgwyngyllgogerychwyrndrobwllllantysiliogogogoch. For obvious reasons, it's usually abbreviated to Llanfair PG.
The 20-volume historical Oxford English Dictionary includes other very long words, most of which are highly technical. These include:
otorhinolaryngological- 22 letters
immunoelectrophoretically- 25 letters
psychophysicotherapeutics- 25 letters
thyroparathyroidectomized- 25 letters
pneumoencephalographically - 26 letters
radioimmunoelectrophoresis- 26 letters
psychoneuroendocrinological - 27 letters
hepaticocholangiogastrostomy - 28 letters
spectrophotofluorometrically - 28 letters
pseudopseudohypoparathyroidism - 30 letters
People sometimes ask whether a DNA string can be considered as the longest English word, given that they can run to many thousands of letters. The answer is no: they're regarded as chemical names rather than genuine words in the sense of meaningful items of vocabulary.
The same is true of the formal names of chemical compounds. These can be almost unlimited in length (for example, aminoheptafluorocyclotetraphosphonitrile, 40 letters) and many contain numerals, Roman and Greek letters, and other symbols, as well as ordinary letters. We don't tend to regard these terms as proper 'words'.