Like 1/4 of the screen. Why?

This one is also easy but not intuitive. There is a red line around the preview with a red circle. If your resolution is high (as it is on a Mac) you need to resize it to fit within the black box inside of the preview window by clicking and holding the red circle in a corner while dragging inward. Once it’s small enough you’ll notice its actually a red box. As you go you may also need to pull the whole red box to fit within the black box by dragging the entire image. You want the red box to match the black box size, unless you’re zooming or have multiple sources.