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.
"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