You can do satellite comms on something that small (maybe an antenna a few centimetres square) if you're only interested in moving a few bits per second or mayb

You can do satellite comms on something that small (maybe an antenna a few centimetres square) if you're only interested in moving a few bits per second or maybe kilobits per second. In the transmit direction you're generally limited by how much power you can pump out (the smaller the antenna, the less antenna gain you have and the more power you need to produce from your transmit chain). In the receive direction, you have a figure called G/T which has to do with the ratio of the antenna gain to the noise temperature of the receiver chain (which includes thermal noise picked up to the universe, plus noise introduced at each gain stage etc.). The simpler the modulation, the less power you need generally, but you can move less data in a given amount of frequency spectrum, so that's the tradeoff.