Hi, you might glean some extra info here:

http://www.cartographersguild.com/showthread.php?t=2596

I think about 300dpi for photo/images is fine but maybe 600dpi for line text and font heavy black and white only images.

That would be a 10800 pixel image wide which is large but not massive and most apps esp PS should handle that fine. You will need a PC with a few gigs of ram to handle that esp if you use a few layers. If you think you need more than a few layers then I would save each layer as an individual image and composite it at the end. Thats just a personal point of advice there which might not be a common way of doing it.

The image canvas size should be the size in pixels or the real size of the map. Note the bit in the tut about what DPI is and how theres a few versions of usage for the word resolution. At end of day the pixel size is the important bit.