A rendered bitmapped image file consists of numerous rectangular pixels (picture elements). The size of a rendered image is measured by the number of pixels it contains horizontally and vertically, and by the aspect ratio of individual pixels (whether pixels are square or rectangular).
Most display devices (for example, a computer monitor) have square pixels, and their Pixel Aspect Ratio is 1. Some devices, however, have non-square pixels (for example, NTSC video has a Pixel Aspect Ratio of 0.9).
To set the pixel aspect ratio, see Set the resolution and pixel aspect ratio.