The Sun is an extremely large bundle of gas more than 100 times the diameter of the Earth and more than 300,000 times more massive.  To get an idea of the size compared to Earth, picture the Sun the size of a Volleyball, the Earth the size of a pinhead, and Jupiter the size of a nickel!  The Sun is large! But then again, there are many things in space way bigger than the Sun! Check out this video to see a comparison of sizes!
Video: Size comparison of Planets and Stars
The Sun's Atmosphere
Question: Describe the properties of each layer of the Sun's atmosphere.
Photosphere Information about the Sun's Photosphere
Chromosphere Information about the Sun's Chromosphere
Picture of the Sun's Chromosphere This is an interesting picture because you can actually see Mercury eclipsing the Sun! This also lets you see how big the Sun is compared to Mercury!
Corona Information about the Sun's Corona
The Sun's Corona: Introduction A good explanation of the Sun's Corona.
Picture showing the Sun's Corona This is taking place during a total solar eclipse.
Arny, Thomas T. (2006) Explorations: An Introduction to Astronomy, Fourth Edition , The McGraw-Hill Companies, Inc. pp5-6, ISBN 0-07-302580-1.