Presumably the limit is based on hardware and the graphics API and not specific to Unity -- the truth is most applications will never need such large images. i.e. if the screen resolution is 1600x1200, then having a texture any larger than that is a waste, since the you can't possibly draw more pixels than there are on the screen. If you need to zoom into the image then you will have to split it up into tiles. That would be better for performance anyway (though you'll probably be able to get away with a much, much smaller texture).
answered Apr 15 '10 at 06:39 PM
It's a Unity limitation. However, only the more recent graphics cards support more than 4096x4096, and there are still a reasonable number of cards in use that only do 2048x2048 at max anyway. So for best compatibility you should use linked textures where each one is no larger than that.
answered Apr 15 '10 at 06:58 PM