A NASA-Goddard concept (circa 2006) for a giant space telescope to be built in orbit using robots. Where Hubble looks fairly solid, this looks relatively gossamer… which is perfectly appropriate given the environment. It would be built in the Earth-Moon L2 position, and then transported to the Sun-Earth L2 position. Robots would provide servicing at the Sun-Earth L2 position, but it would be brought back to Earth-Moon L2 for major servicing. Launch would require three heavy lifters like the Ares V, or a single launch of a greatly uprated vehicle (4 5-segment solid rocket boosters, a large upper stage and a total length of 415 feet). Mass of the scope is about 200 metric tons.
As far as capability, a scope like this – operating in the UV-through-visible-to-near-IR wavelengths – could resolve individual Sun-like stars as far out as the Coma cluster (321 million lightyearsaway), or spot exosolar planets out to 100 parsecs.
It was big.
The foil-based sun shield was big enough that photon pressure would produce a noticeable torque on the structure, so the scope was given a solar sail “tail” to counteract this.