Photoshop has a 2GB file size limit, as well as a 30,000 pixel limit in either dimension.
#1
In Photoshop 7, the limit is 30,000 pixels in any one dimension. For instance, a 29,999 pixel by 29,999 pixel image is theoretically possible (as long as the file size does not reach 2GB), but a 30,001 by 400 pixel would run afoul of the limit.
In other words, if your image is taller or wider than 30,000 pixels you're in trouble. If the file size reaches 2GB, regardless of pixel dimensions, you're in trouble too. Sort of a double limit, really.
#2