From the NASA description:"The James Webb Space Telescope will be a large infrared telescope with a 6.5-meter primary mirror. The telescope will be launched on an Ariane 5 rocket from French Guiana in October of 2018.
JWST will be the premier observatory of the next decade, serving thousands of astronomers worldwide. It will study every phase in the history of our Universe, ranging from the first luminous glows after the Big Bang, to the formation of solar systems capable of supporting life on planets like Earth, to the evolution of our own Solar System."
By comparison, its predecessor, the Hubble Space Telescope, had a 2.4m mirror and was situated in an orbit 570 km from the Earth.