9000 x 9000 in P5. However, I should point out that it was a trivial scene: a sphere on a plane, both with procedural shaders and reflection. Mostly I was doing it to see if there was a hard-coded upper limit on rendering size. I figure 30x30 at 300dpi was good enough. Now, with real scenes, the biggest was 4500 x 4500, and I had to do tricks with that by doing multiple passes with some of the items removed and then composite it all back together. (See "The Day Her Replacement Arrived" in my gallery.) Usually I just do normal screen resolutions, and then if I need a big print, I go for 150dpi (or less) and then scale up via Photoshop with a bicubic scaling filter.