I would expect this has been covered previously but I couldn't find anything with a quick search. If I try to save a large image over around 3000x3000 pixels as a .PNG file then it gives me an error if I select 8 bit (or leave it on auto detect), but works fine with 24/32 bit. It will do this with a blank canvas or a simple drawing, but if I paste a portion of an image with lots of colors on the canvas before saving, then suddenly it will work (even at 8 bit).
I am running Windows 7 64-bit with 4 GB RAM.