On my display the guide projector cuts off on the sides, changing gui size only scales the content, not the external bounds of the guide, which seem to be a fixed aspect ratio. For reference my screen is 2256 x 1504, a 3:2 aspect ratio, which is fairly uncommon, most displays are 16:9, which appears to be what you've fixed the guide projector at.
On my display the guide projector cuts off on the sides, changing gui size only scales the content, not the external bounds of the guide, which seem to be a fixed aspect ratio. For reference my screen is 2256 x 1504, a 3:2 aspect ratio, which is fairly uncommon, most displays are 16:9, which appears to be what you've fixed the guide projector at.
Image for reference to what I'm talking about: