Currently the max size of local cover images is hard coded to 600 x 800 px. Libraries with less resources might want to have smaller images, especially since it can get big real fast (see bug 17650). The max width and max height could be specified in a system preference rather than be hard coded. To test : 1. Turn on LocalCoverImages 2. Attach a large image file to a record 3. Right-click on the image and download it --> It should have been reduced to a maximum of either 600 (width) or 800 (height) px
See Bug 33857 The patch add a script that reduce and resize local cover images. The max width and max height are not specified in a system preference, but the image is no size limit to 600 x 800 px.