As an agglutinative language, Turkish allows the construction of words by adding many suffixes to a word stem. The longest word in the Turkish language used in a text is muvaffakiyetsizleştiricileştiriveremeyebileceklerimizdenmişsinizcesine which has 70 letters. It is derived from the noun muvaffakiyet ('success') and means "as though you are from those whom we may not be able to easily make into a maker of unsuccessful ones". It was used in a contrived story designed to use this word.[1][2]
Not considering suffixes, the longest Turkish dictionary words have 20 letters: These are kuyruksallayangiller (the biological family Motacillidae), ademimerkeziyetçilik ('decentralization') and elektroensefalografi ('electroencephalography'). In comparison, the word muvaffakiyet has 12 letters, so it should be possible to use various other suffixes to make an even longer word from these ones. Therefore, -sizleştiricileştiriveremeyebileceklerimizdenmişsinizcesine can be added as a suffix to any meaningful Turkish noun. For example, ademimerkeziyetçiliksizleştiricileştiriveremeyebileceklerimizdenmişsinizcesinedir is another possible longest meaningful word in Turkish with 81 letters, which means "as though you are from those whom we may not be able to easily make into a maker of decentralizationless ones".
There is no principled grammatical reason for not being able to make a Turkish word indefinitely long, as there are suffixes that can act recursively on a word stem. In practice, however, such words would become unintelligible after a few cycles of recursion.