I mean, a plain JPEG can tolerate up to I think 2^16 × 2^16 pixels, and already that you don’t really want to decode from a single unseekable bitstream with no index and no effort to improve locality of data required to fill a rectangular viewport. [ImageMagick’s display(1) is the best at tolerating huge JPEGs and even it, IIRC, conks out after 2^15 × 2^15.] You can allocate however many bits you want for the size, but beyond a few dozen megapixels you really need to do indexed independently-decodable tiles, and when the image is dozens of gigabytes after compression, you need a pyramid of pre-downscaled versions as well (1/4 + 1/16 + ... ≈ 33% overhead which is completely acceptable). Thus my question.