This information is listed in the Help file. It appears when you select the 'Image file' field on the 'Edit Item Window - Image tab' Help screen.

Here are the technical details:
The display area for the image is 277 x 397 pixels; optimally, images should be approximately that size. If they are larger than that, the image will be shrunk to fit the available space, maintaining the aspect ratio of the image. If you have images that are smaller than 277 x 397 pixels, the image size will be maintained; it will not be enlarged to fill up the available space.