When 3 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, then we use 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
How big is the cropped image?
The actual size of the Thumbnail image will depend on the Crop that you have 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 is the case then the cropped version for the Carousel would be 437x500 px.