IIRC, a 16:9 (1.78) aspect display with 1080 (or 1100) line pairs the threshold of the human eye being able to resolve said resolution is 1.5x the screen's width. Let's put this into an example:
I have a pair of 50" 1080P plasma displays. The 50" is the diagonal so the width portion is 50 x 0.8716 or about 43.6". Now, 1.5 times this and put into feet is about 5.45 feet viewing distance. These results are proportional so any screen large or small is a simple fraction of this distance.
Also keep in mind that if your sight is better/worse than 20/20 then that is a factor, too. So, how many of you are sitting at your display's threshold? Sitting beyond it means you may have paid for something you cannot see, and sitting closer than said threshold means you are on meth (kudos).