Ideally, post-zip channel size should be 500k or less, preferably as small as possible. If you are submitting a public channel, please do your utmost to keep the size low. Use Photoshop's Save for Web option to optimize for the best quality image at the lowest file size. Make sure no extraneous gunk like DS_Store and thumbs.db, any hidden or system files, zips of older versions of your channel, etc are not accidentally zipped up, bloating your channel.
- Joel