An utterly emormous construction on an unthinkable scale. A mere theory, a Dyson Sphere is a "space ship" that encircles an entire star, so as to harvest ALL of its energy. To make a Dyson Sphere would provide an unimaginable amount of living space and energy.
A lot of debates have been started over the size of a Dyson Sphere, but for a small(ish) star the diameter would have to be close to 200,000,000 km. Even though many people also debate whether there is even enough matter to MAKE a Dyson Sphere, almost everyone agrees that if a civilization were to have the ABILITY to make one, they wouldn't even have a use for a Dyson Sphere, as by that point they probably would have had something better.
A Dyson Sphere for the solar system would be about 600 million times the surface area of the Earth.
A much debated topic, estimated sizes of Dyson spheres can be from 190000000 km to several light months (for more than one star inside of the sphere).
"originally proposed in 1959 by the astronomer Freeman Dyson as a way for an advanced civilization to utilise all of the energy radiated by their sun. It is an artificial sphere the size of an planetary orbit." -Anders Sandberg