QUESTION:
How big is IO and how big are the volcanoes?

ANSWER from Clark Chapman on January 22, 1996:
Jupiter's innermost major moon, Io, is a little larger than our own moon, 
more than 2000 miles in diameter.  It has hundreds of volcanic
caldera (collapse craters) larger than 10 miles across, and some 
are 10 times as big.  

The plume of volcanic ejecta, driven by sulfur chemistry, can rise 
200 miles or more above the surface of the colorful moon.