According to Donald Menzel (he is the earliest source of this information I know of), the term "flying saucer" first was mentioned in '47 and the traditional shape followed on, although in reality most of these "objects" are not very saucer-like. Here is some write-up on this story I found here:
On June 24, 1947, an amateur pilot named Kenneth Arnold was flying a
small plane near Mount Rainier in Washington state when he saw
something extraordinarily strange. Directly to his left, about 20 to
25 miles north of him and at the same altitude, a chain of nine
objects shot across the sky, glinting in the sun as they traveled.
By comparing their size to that of a distant airplane, Arnold gauged
the objects to be about 45 to 50 feet wide. They flew between two
mountains spaced 50 miles apart in just 1 minute, 42 seconds, he
observed, implying an astonishing speed of 1,700 miles per hour, or
three times faster than any manned aircraft of the era. However, as if
controlled, the flying objects seemed to dip and swerve around
obstacles in the terrain.
Arnold's sighting was "such a sensation that it made front page news
across the nation," UFO-logist and author Martin Kottmeyer wrote in an
article ("The Saucer Error," REALL News, 1993).
"Soon everyone was looking for these new aircraft which according to
the papers were saucer-like in shape," Kottmeyer continued. "Within
weeks hundreds of reports of these flying saucers were made across the
nation. While people presumably thought they were seeing the same
things that Kenneth Arnold saw, there was a major irony that nobody at
the time realized. Kenneth Arnold hadn't reported seeing flying
In fact, Arnold had told the press that the objects had flown
erratically, "like a saucer if you skip it across the water." They
were thin and flat when viewed on edge, he said, but crescent-shaped
when viewed from the top down as they turned. Nonetheless, a reporter
named Bill Bequette of the United Press interpreted Arnold's statement
to mean that the objects he saw were round discs. According to
Benjamin Radford, UFO expert and deputy editor of the Skeptical
Inquirer, "It was one of the most significant reporter misquotes in
"The phrase 'flying saucers' provided the mold which shaped the UFO
myth at its beginning," Kottmeyer wrote. UFOs took the form of flying
saucers, he noted, in artist's renderings, hoax photos, sci-fi films,
TV shows and even the vast majority of alien abduction and sighting
reports for the rest of modern history, up until the present day.
This story is described in Menzel's book "Flying Saucers", and he claims that there is no mention of such objects before that incident. Therefore, there is no sense in trying to explain the shape, as it was not caused by logic, but rather by pure accident of the craze starting with this term spread through media.