ImageSize is an option for Export, Display and other graphics functions, as well as for Cell, which specifies the absolute size of an image to render.
ImageSize->x specifies that the image should have a width of x printer's points.
ImageSize->72 xi specifies that the image should have a width of xi inches.
ImageSize->x, y specifies that the image should be rendered within a region x printer's points wide by y printer's points high.
The image will fill the region only if its aspect ratio is exactly y/x.
In Display and other graphics functions, the default setting for ImageSize is Automatic. This specifies that when output is sent to the front end, the front end should determine the size of the image. When output is sent elsewhere, the effective default is 288, corresponding to 4 inches.
In the front end, the typical default setting for ImageSize is also 288, corresponding to 4 inches.
See The Mathematica Book: Section 2.9.19 and Section 2.10.12.
See also: ImageResolution, ImageMargins, AspectRatioFixed.