Inspired by a discussion at Herculodge about the size of the ferrite bar antennas used for AM radios, I decided to measure the antennas on a few of my AM radios.
First, the Zenith Royal 76 (Symphony), which is very good with respect to sensitivity. Its ferrite clocks in at 6 1/4 inches. The Royal 76, and its predecessor, the Royal 820, were top-of-the-line models.
Next, an earlier, mid-range Zenith radio, the Royal 670. It's not quite as sensitive as the Royal 76. Its bar is 5 1/4 inches long. However, it also has cheaper circuitry than the Royal 76.
Finally, the startlingly good Westinghouse H950PN10, also with very good sensitivity. It has a 7-inch ferrite bar, *plus* an additional longwire antenna that appears to be permanently attached to the radio.
None, though, matches up to the 8-plus-incher described in a Herculodge article about the Panasonic RF-1600. And I'm afraid to take apart the case of any of my GE Superadios (because their cases are notoriously difficult to put back together without breaking something), but I wouldn't be surprised if they had long ferrites, too.
Does the length really matter? It's not the only variable with these radios; circuitry matters, too. But might it be an indication of a radio's ability to pick up weak AM signals? Possibly.