This is a great interactive planet size comparison page:

http://www.sciencenetlinks.com/interactives/messenger/psc/PlanetSize.html

http://spaceplace.nasa.gov/en/kids/sse_flipflop2.shtml

http://www.windows.ucar.edu/tour/link=/our_solar_system/relative_size.html

http://www.freemars.org/jeff/planets/planets5.htm

http://hyperphysics.phy-astr.gsu.edu/HBASE/solar/sizes.html

http://www.rense.com/general72/size.htm

Order of Size – Largest to smallest

Jupiter - 88,700 miles (142,800 km) in diameter

Saturn - 74,898 miles (120,536 km) in diameter

Uranus - 31,690 miles (51,118 km) in diameter

Neptune - 30,775 miles (49,528 km) in diameter

Earth - 7,926 miles (12,756 km) in diameter

Venus - 7,521 miles (12,104 km) in diameter

Mars - 4,222 miles (6790 km) in diameter

Mercury - 3,031 miles (4,878km) in diameter

Pluto - 1,413 miles (2274 km) in diameter

Distance from the sun

Mercury - 36 million miles

Venus - 67.2 million miles

Earth - 93 million miles

Mars - 141.6 million miles

Jupiter - 483.6 million miles

Saturn - 886.7 million miles

Uranus - 1,784.0 million miles

Neptune - 2,794.4 million miles

Pluto (a dwarf planet) - 3,674.5 million miles

Good luck on your science project