Edit 3: I belive I went down the wrong path on より in this comment, and came pu with something incorrect. I’ll leave it here, but please see my follow up response a couple of posts down as well.
Here’s how I break it down:
“The length of the break and tail feathers are included in the (animal) body length.”
“Because the length of the break and tail feathers are included in the (animal) body length, …”
“The time of standing” (“When standing”)
“From” (like another meaning for から)
“From the time of standing…”
Edit: I think this より can also be translated as “after” rather than “from” here, so another reading would be “after the time of standing”.
Edit 2: The book “All About Particles” (Naoko Chino) mentions, “indicates a cause, reason, or motivation” as another meaning for より (“as a result of”). Maybe that’s more accurate for this usage? “As a result of (when) standing”? If anyone can pinpoint this より, don’t hesitate to clarify!
“It becomes much longer.” (“It will be much longer.”)
“Because the length of the beak and tail feathers are included in the (animal) body length, at the time (a penguin) stands, it becomes much longer.”
The gist of the conversation (if I’m correct) thus is: “People expect penguins to be taller than they are. This is because the recorded length is from beak to tail feathers. When a penguin stands up, it becomes taller, more closely matching the recorded length. But when it’s not standing up, it’s much shorter than the recorded length, which is why people find penguins to be shorter in person than the expected from reading the recorded length.” (Somehow, my “gist” become quite long.)