In the August issue we exhibited a progressive terminal transaddition where the single letter S is extended by one letter at a time up to the 13 letter SHEPHERDESSES, with all terms in the series being legitimate words (most found in dictionaries).
By stretching logological licene somewhat, a longer chain is possible culminating in the 18-letter word CHARACTERISTICALLY.
Word Ways: Vol. 42
, Article 24.
Available at: http://digitalcommons.butler.edu/wordways/vol42/iss4/24