~ 21 inches
"The MK 28 was one of the longest-serving and most common
thermonuclear weapons in the U.S. arsenal until it was eventually
replaced by B61 and B83 bombs. It was the first U.S. nuclear warhead to
be designated as a complete weapon system.
MK 28 warheads, fuses, and shape components could be assembled
into five different "drop shapes" resulting in a range of
sizes, configurations, and yields called the "Building Block"
principle. The W-28 warhead
was used in the USAF HOUND DOG air-to-surface and MACE