How big external monitors will a Macbook Pro Retina support?
Here's one example of someone running two 27" displays at 2560x1440 via Thunderbolt/DisplayPort plus an HDMI display at 1920x1200:
http://blog.macsales.com/14241-macbook-pro-15-with-retina-display-can-run-3-external-displays

The Thunderbolt ports officially support up to 2560x1600 so that resolution shouldn't be any problem either:


  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
  http://www.apple.com/macbook-pro/specs/


To get up to that resolution, you're limited to either Thunderbolt/MiniDisplayPort or Dual-Link DVI via an adapter - single link DVI and VGA adapters won't go that high.

For the HDMI port, I haven't seen any specific maximum resolution. This person got up to 2048x1280: http://reviews.cnet.com/8301-3121_7-57460642-220/how-to-power-four-simultaneous-screens-with-the-retina-macbook-pro/

Generally, external monitors work fine in Windows. I see no reason why it would be any different on the Retina model or on Windows 8 specifically.