Use the controls in the Previews preference pane to set how Aperture creates and displays preview images.
The Previews preference pane contains the following controls:
“New projects automatically generate previews” checkbox: To set Aperture to create previews for the photos in all new projects and new libraries, select this checkbox. Deselect this checkbox if you don’t want previews created for photos in new projects.
“Share XML with other applications” pop-up menu: Choose whether to share JPEG preview images via XML as needed for other applications.
Note: Some applications share preview images using the media browser, which does not require XML to access the Aperture library.
Photo Preview pop-up menu: To set a maximum pixel size for your previews, choose a size from this pop-up menu.
“Photo Preview quality” slider: To change the image quality of the previews, drag the slider. The higher the quality setting, the more disk space the previews require.