I don't know what the highest wave size on record is, but I do know that in a 6/6 AI 45-minute Defender Mode game on a 120-planet map where I chose to start with 118 worlds, the first wave was about 50,000 strong. (I think it was a 45-minute game, anyways, but I'm not positive). That convinced me that I should perhaps actually pay attention to the suggestion in the tooltip for the mode and not start in control of ~100% of the galaxy.