“Length” and “size” and “distance” all measure the same physical dimension, but they are subtly different:
“length” refers to a span across space
“size” refers to the largest spatial dimension of an object
“distance” refers to how far away something is from something else (center to center)
Diameter is used for size, because it is closest in spirit to “length”. For a cube-like solid, the size is just the magnitude of the longest side. [The diagonal of a cube would be ^0.2 longer, so it generally won’t make a difference.]
Translating from mag diameter to another mag quantity uses simple logarithmetic. To convert to:
radius: ^-0.3 size
circumference: ^0.5 size
surface area: 2 x size
volume: 3 x size
For example, Earth is “^7 size” and to walk all the way around it would be “^7.5 distance”. And it has “^14 surface area” and “^21 volume”.
The Moon, Earth, Jupiter, and Sun are ^6.5, ^7, ^8, and ^9 size, respectively. The distance to the Sun (1 au) is ^11, and the distance to Voyager is ^13.
How far is the Moon? We know that it takes seconds (^0 time) to radio the Moon, and since the speed of radio is the speed of light at ^8.5, so the Moon is ^8.5 distance from Earth.
Conversions
^-10: Angstrom
^3: mile/kilometer
^11: Astronomical Unit (AU, distance from Earth to Sun)
^16: light-year (^8.5 speed of light + ^7.5 year)
Mag Size
^-10 (1 ångström): Hydrogen atom
^-9 (nm): width of DNA/nanotube
^-8: cell wall
^-7: HIV
^-6 (µm): bacterium
^-5: red blood cell; width of human hair
^-4: water bear
^-3: flea
^-2: coin
^-1: banana
^0: family home
^1: large building (school)
^2: huge building
^3: Three Gorges Dam; smallest moons and asteroids
^4: city; asteroids
^5: state or minor country or minor moon; largest asteroids