Crop sizes are used when users check out files from an archive using FotoWeb Desktop. As an administrator you can choose the sizes and aspects that the user can choose from when cropping pictures.
Open the archive properties and go to the Export and Import tab. Locate the Cropping section and choose a crop profile set from the drop-down menu. If you also want users to be able to choose their own sizes and store those changes in the crop profile for later use, tick the checkbox labeled Allow crop profile changes.
Learn more about creating crop profiles.