In the Image Texture Edit Window, Click on the Map button at the top; it will open the Image Map dialogue box. Make sure View Scale is at 100%, then click on the map, Copy, and Paste into a new Photoshop document.
For completeness, let me mention that you can also extract really large image maps in the Diffuse channel by attaching them to a simple primitive and then exporting it as a VRML. This generates an object and a texture file. Thanks to Jean for that suggestion.