I have a friend who is a certified ISF technician and has calibrated 1000's of televisions. I stand by his advise. At 50" or less there is no difference to the human eye between 720 and 1080. Larger than that and the difference MAY be apparent, or not depending on your distance from the screen.
I believe you meant to say that at 50" or MORE there is little difference and that differences may be apparent at LESS than that distance. While I'm not sure that 50" is the threshold, the differences in higher resolution picture quality, i.e. 1080p v. 1080i or 720p, are much more discernable at closer viewing distances.