Again, a beautiful object, whether it be a living organism or any whole composed of parts, must not only have an orderly arrangement of parts, but must also be of a certain magnitude; for beauty depends on magnitude and order. Hence a very small animal organism cannot be beautiful; for the view of it is confused, the object being seen in an almost imperceptible moment of time. Nor, again, can one of vast size be beautiful; for as the eye cannot take it all in at once, the unity and sense of the whole is lost for the spectator; as for instance if there were one a thousand miles long. As, therefore, in the case of animate bodies and organisms a certain magnitude is necessary, and a magnitude which may be easily embraced in one view; so in the plot, a certain length is necessary, and a length which can be easily embraced by the memory. The limit of length in relation to dramatic competition and sensuous presentment, is no part of artistic theory. For had it been the rule for a hundred tragedies to compete together, the performance would have been regulated by the water-clock,—as indeed we are told was formerly done. But the limit as fixed by the nature of the drama itself is this: the greater the length, the more beautiful will the piece be by reason of its size, provided that the whole be perspicuous. And to define the matter roughly, we may say that the proper magnitude is comprised within such limits, that the sequence of events, according to the law of probability or necessity, will admit of a change from bad fortune to good, or from good fortune to bad.
Which is important with series that are just flowing on and on with the same tale.
Trilogies can provide a nice structure.
Indeed, I have notice a number that use the three books to form an elegant structure:
Book 1 covers the main character discover the central issue and its depth.
Book 2 has the main character (and friends) scuttle about trying to survive this knowledge
Book 3 has the serious counter attack start and come to fruition, ending the problem.
Elegant, precise, and giving you a useful conclusion, even if cliff-hangerish, to the first two, by resolving something. (The precise activities are concentrated, not exclusive to each book, of course.)