Io is 1.04 times the diameter of the Moon. [see page for the Moon]
Io is .756 times the diameter of Callisto (a satellite of Jupiter).
Io is 2.25 times the diameter of Titania (a satellite of Uranus).
Io is .536 times the diameter of the planet Mars.
Io is 3.46 times the diameter of Tethys (a satellite of Saturn).
Io is .285 times the diameter of Earth.
Io is 7.5 times the diameter of Miranda (a satellite of Uranus).
Io is 16.5 times the diameter of Phoebe (a satellite of Saturn).
Io is .072 times the diameter of the planet Uranus.
Io is 41.1 times the diameter of Pandora (a satellite of Saturn).
Io is .031 times the diameter of the planet Saturn.