What is the maximum resolution for a second monitor attached to a MBP Retina?
From the Tech Specs page over at Apple:

13":


  Dual display and video mirroring: Simultaneously supports full native resolution on the built-in display and up to 2560 by 1600 pixels on up to two external displays, at millions of colors


15":


  Dual display and video mirroring: Simultaneously supports full native resolution on the built-in display and up to 2560 by 1600 pixels on up to two external displays, at millions of colors