Just as in English, you can "stretch" the adjective microscopic (11 letters) into a 16-letter word by adding the prefix ultra-, and then turn the adjective into an adverb with the suffix -ally, for a grand total of 20 letters: ultramicroscopically.
You could even stretch this further with "nonce formations" like deultramicroscopicalization (27 letters) -- which, in a science-fiction context, could mean something like "the reversal (de-) of the process of making (-ization) something much much smaller," as happened at the end of Honey, I Shrunk the Kids.