The Loupe will give you a full resolution print preview (actual print size) of anywhere you hover on the page. The small square is the preview area and the large square is the actual full resolution preview.
Above the spread in Designer View, you'll see a magnifying glass icon.
The Loupe will pop up and start creating the full resolution preview of your spread. This may take a couple of seconds. You’ll notice that once loaded, the preview is instant. You can move the preview square around your spread and view a magnified version at full print resolution. This allows you to verify sharpness of your images at resolution, as well as check alignment and spacing.
You can see a video demonstration of the High Res Loupe here.