Like any lens, the SGL forms an image by focusing light from a distant source. The location of that image is determined by the focal distance of the lens.
For the SGL, this distance is not fixed. The lens is "blurry": different rays of light are focused at different distances from the lens, beginning at 550 times the Earth-Sun distance, the astronomical unit (AU). Realistically, we need to be at least 650 AU from the Sun, however, in order to be able to distinguish light focused by the SGL from light from the Sun itself or the solar corona.
This one number, 650 AU, already gives us a lot of information about the geometry of the SGL.
Let's say, we have a solar system containing an Earth-like planet of interest 10 light years from the Earth. Ten light years is approximately 100 trillion kilometers. So that is where our "exo-Earth" is located.
The image, in turn, forms at 650 AU from the Sun, which is approximately 100 billion kilometers. This is 1/1000th the distance from our solar system to the target. Therefore, basic geometry tells us that the image of the exoplanet will be 1/1000th in diameter compared to the actual planet.
The diameter of the Earth is approximately 13,000 km. That means that the image of an exo-Earth, situated at 10 light years, will be approximately 13 km wide at 650 AU from the Sun.