The Loupe tool will give you a full resolution print preview (actual print size) of anywhere you hover on the page. To access this feature, open your spread in Designer View, then click the magnifying glass icon in the top toolbar.
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.