What Is The Order Of The Planets, By Size?

Although your question doesn't specify, I assume that you are only referring to the planets within our own solar system, - and not to the enormous number of other planets that have so far been discovered in the wider universe!

The Planets, Ordered by Diameter:

• Jupiter - 143,000 km
• Saturn - 125,000 km
• Uranus - 51,000 km
• Neptune - 49,000 km
• Earth - 12,800 km
• Venus - 12,100 km
• Mars - 6,800 km
• Mercury - 4,900 km
• Pluto (for comparison) - 2,300 km
  

The Planets, Ordered by Distance from the Sun:

• Mercury - closest to the sun, and also the smallest planet
• Venus - the hottest planet, due to its thick atmosphere
• Earth - the largest of what are referred to as the "inner planets"
  
• Mars - the planet most likely to be capable of supporting life as we know it
• Jupiter - by far the largest planet in the solar system
• Saturn - most famous for its rings, although it's not the only planet to have them
  
• Uranus
• Neptune - at lows of -210C, Neptune is the coldest planet. Its orbit averages 4.5 billion km from the sun
• Pluto - no longer considered to be a planet

Number of Planets in the Solar System
There are now considered to be just eight planets in the solar system, since Pluto was demoted to the status of dwarf planet in 2006. There is considerable debate over this matter, however, and as current knowledge stands, there are four other dwarf planets in the solar system besides Pluto.

Eris (originally named Xena) is the largest of the dwarf planets, and was itself almost pronounced a planet, until astronomers decided in 2006 that it should be classified as a dwarf planet. They reclassified Pluto at the same time.