We worked with single steps of 1024x768 pixels size with a center part of 50% left out. An illustrator would either work inwards or outwards of the existing portion. Outwards that would give a photoshop document of 1024x768 with the previous step locked on a centered layer of 512x384. Inwards with the upscaled previous image of 2048x1536 as the frame, and the respective centered paintable area of 1024x768.

The illustrators then could easily blend their work into the other peoples artwork. The whole thing evolved very un-planned. There was no guideline as to what should be depicted. Illustrators just had to pick up what others have left for them. Best part of the process for me.