the square shape appears to have been introduced as a result of the tendency of un-optimised windows environments to use so many menu lines and tool bars and fixed dock that you need the square to have a normal rectangle left for the actual material

1440x900 resolution on LCD screen (speaking from mac end with nothing on screen except basic menu bar and collapsed window header on browsers) seem about OK and allows easy viewing of 1024x768 images and slightly larger at a pinch. The actual size is less important than the resolution, but for desks 20" nominal about the smallest worth bothering with (though my mac laptop does that resolution in 15.4")