Okay I'll do the math, if we look at a chart we can find that most human heads fit within the measurements of 6.5 inches wide, and 9.4 inches tall [1]. That fits nicely within the 8x10 inch standard size for photographs that is common, so assuming we display these images at the maximum resolution human eyes can discern [2](somewhere <300 pixels per square inch) that means that in order for you to display a human head life-sized at 'visually flawless' resolution only requires an image with a resolution of 2400x3000 pixels.

I can see the appeal of life-sized photos on social networking sites in the future. Group photos could benefit from being larger than that to include multiple people at life-size, but I'm unsure of how a resolution greater than 2400x3000 would be useful for a profile picture at this point.

[1] http://upload.wikimedia.org/wikipedia/commons/6/61/HeadAnthr...

[2] http://prometheus.med.utah.edu/~bwjones/2010/06/apple-retina...