Astrology has a much wider definition of a planet than astronomy: it can be any heavenly body that appears to orbit the Earth, including the Sun and the Moon, and size or distance have nothing to do with it. What is important in astrology is the kind of influence a body emits and how it affects life on Earth. Earth too is a planet, although it has yet to be recognized as such by astrologers. The table below shows the great variance in sizes(thanks to astronomy) among the main planets accepted in astrology thus far.
As you can see, the sizes vary considerably, from the huge Sun in the background to tiny Pluto, yet each is equally important in the scheme of astrology. Astrology's ideal is to have twelve planets -one for every sign of the zodiac. The above diagram contains eleven. One more must be determined.