How Large Can a Telescope Be?

From ESOCast: on clear nights we can look up at the stars and marvel at the vastness of the universe. Our eyes quickly adapt to the dark. As our pupils widen, more light streams onto our retinas and fainter stars become more visible. But the light-collecting area of the human eye is tiny. To peer much deeper into the night sky astronomers need telescopes with enormous primary mirrors. How large does modern technology allow us to build telescopes? How far into space can they see?