Yeah there should be a warning. Although if you exceed 2k by 2k atlas, you really should reconsider what it is you're putting in there. Take advantage of 9-slicing on sprites and consider moving sprites larger than 256x256 outside of the atlas, leaving them as plain textures. Also note that Unity's packer is terrible at packing long or narrow sprites. So something like 1000x20 will waste a lot of space.