One person pegs it at 10,000

[spdrun]

One person pegs it at 10,000 square miles or 100 x 100 square miles.

This assumes no conservation and no other clean sources of power like hydroelectric:

http://www.energytrendsinsider.com/2008/02/25/running-the-u-s-on-solar-power/

Here are some calculations for rooftops:

http://www.energytrendsinsider.com/2007/07/27/a-solar-thought-experiment/

It’s actually somewhat doable. You’d need a storage mechanism like pumped hydro or molten salt though.