Pop songs are 200 seconds long because the mechanical properties of 78 and 45 rpm records can deliver one song with decent fidelity of that length. They can’t handle ten minutes, and one minute is too short to charge for.
The number of books carried by a local bookstore was the right amount to balance paying the rent and satisfying most customers. And the number of books published reflected the fact that the only way to get a new book in was for the store to take one out.
Movies are long enough to justify buying a ticket, but not so long that the theater can’t have multiple showings.
Books are around 350 pages because pamphlets are too hard to sell and books that are too long are hard to bind and manipulate.
Sitcoms are half an hour long because two sitcoms an hour maximizes the possible audience more than extending one to double the length might.
The newspaper is the length it often is because the editors are balancing the time each subscriber can spend with it against the publisher’s desire to sell the most profitable number of ads. “All the news that fits.”
When technology changes the media, when distribution and consumption shift, the definition of just the right length shifts as well. Podcasts changed the length of interviews, Linkedin changed the length of a resume and YouTube changed the length of funny videos… the cycle continues.