When three images are shown in the Carousel (using the Showcase layout), we use the Widescreen crop for cover image (16:9) image for the larger panel, and the Square crop for thumbnail version for the smaller panels.
However the panels aren't actually square, they're rectangle, so we crop the sides of the square slightly.
Please see the attached .jpeg & .psd templates at the bottom of this article.
How big is the cropped image?
The actual size of the thumbnail image will depend on the crop you've selected from the original image during content creation. However if you do not resize the selection box the thumbnail will be 500x500 px. If that's the case, the cropped version for the Carousel would be 437x500 px.