We have studied statistical distributions of the frequency of sentences over their length for Ukrainian, Russian and English corpora and found the average sentence lengths in terms of linguistic signs, letters and words. It has been shown that the tails of the statistical distributions are satisfactorily described by the exponential function or the related ones, which is consistent with random nature of the sentence length. We have proven that the fluctuations of the frequency of sentences of different lengths depend on the average values of that frequency according to the Taylor’s power law. Significant relative fluctuations of the frequency and the relative changes in the average sentence length confirm the importance of fluctuation phenomena in the statistical linguistics.