What is the maximum size of content the human eye can focus upon? What is the maximum size of content the human eye can focus upon on a computer screen?

Is there a general equation that gives this size as a measure of distance from the screen etc.?

What is the shape of this focus area- is it a square? a circle?
The fovea (highest-resolution area of the retina) can only "see" the central two degrees of the visual field; the field of foveal vision is approximately circular, although it varies slightly by individual.  This depends on the distance from the eye or sensor.  This is called "visual angle" or "angular size", and it can be calculated using the formula



where S is the object's linear size and D is the distance between the eye/sensor (sometimes called point O) and the object.

More here: http://en.wikipedia.org/wiki/Visual_angle 
Calculator here: http://www.oocities.org/robertellis600/va.html