Viewing distance plays a major factor as well. There's a bunch of charts online that do the math for you...here, for example:
Chart: 1080P Does Matter - CarltonBale.com
Sitting 8 feet away is pretty common - in which case, 1080p on a 60" screen would clearly look different than 720p. If you sit any closer, a 50" will do, and if you're insane (or Japanese), a 42" will do the job.