Excellent question. The answer is, "They can't." It's impossible to accurately estimate the size, distance and speed of any object moving against the sky with no other reference points. For comparison, just last week, a zeppelin-like device accidentally went adrift over Salt Lake City and sparked mass UFO reports. The general consensus of the witnesses was that it was over 100 feet long. But we later discovered exactly what it was and who had made it. Turned out it was less than 30 feet long. But I'm sure this UFO really was exactly a mile wide.
BTW, I'm a longtime member of a Skeptics group, was the editor of their newsletter for several years and have written for CSICOP's Skeptical Briefs. I guess I just enjoy hanging out with ignorant, rabid, mean-spirited, hostile, assaultive, uninformed, clueless skeptics. You know, university physics professors, engineers who've worked on government projects such as the superconducting supercollider, a top level FAA airline crash investigator, morons like that. Good times!
Hwy, I’ve got a pile of Skeptical Inquirers up in the attic. They were fun.
I like to ask people which is bigger, the sun or the moon. And then have them explain their answer.