Every sentence is a unit of energy, moving from one point to the next. The sheer length of a sentence is rarely the cause of a lack of clarity; instead, it's more likely to be flaws in the structure of the sentence as it evolves. The energy in a sentence might peak in the second clause, for example, and then dither along with several inessential details, expiring well before the period. The sentence may force the reader's eye into painful contortions while it leaps from conjunction to conjunction, the reader having to work too hard and suffer too much to keep up.
That said, long sentences are more likely to lose the reader if not effectively structured. A short sentence that lacks focus and loses the reader ends quickly, giving the reader a breath to catch up by the next sentence. A long sentence that goes awry can lose a reader for lines at a time, making it that much more difficult to catch up. The writer can take this into account by making sure every clause in an extended sentence, every detail given, leads the reader inexorably and clearly on to the period.