The Size parameter category contains the Zoom Factor slider. This slider controls how far you zoom into or out from the source image.

The Zoom Factor ranges from .1 to 20. At .1, the application scales down the image to one-tenth the original. At 1, the application applies no magnification to the source image. At 20, the application scales up the image 20 times - that is, by 2,000%.

The larger the Zoom Factor, the smaller the field of view rectangle appears when you use Source mode.