I hope kuja.girl doesn't mind, but I did similar (with KG-Chart) - 4 parts to the pattern, with a stitch count of 1000 x 798.
As far as I understand (someone correct me if I'm wrong about it) but the only difference the count per inch would (or at least, should) do is make it into a smaller final result - which makes it easier in the sense that you are less likely to need to combine parts (or get custom fabric).
As said, on 18-count, it would be 55.56 x 44.33 inches. (On 14-count, 71.43 x 57")
20-count puts it at 50 x 39.9"; 22-count at 45.45 x 36.27 (which seems almost ridiculously reasonable)
I didn't limit the number of colors (if you're doing it this big, I figured that you'd be willing to do as many colors as it suggested), so I'll have the color chart here once I work it out: (798,000 total count - 234 colors)
EDIT: And Color Chart (or, at least, list) is here:
I can put PDFs in another post, but for now, a combined picture from the PDFs (the larger parts have 72 pages): Note that the line of lighter colors shouldn't actually appear, but is needed in the picture to have the quarters in the correct spots.