With all Fabrik themes, control the crop ratios of your thumbnail/list images for most layouts from the Layouts area. Navigate to Customise > Theme Layouts. In each layouts group, tap the palette icon over the layout to reveal the options drawer.
Our themes are designed to display meaningfully across more than 100 screen sizes between mobile (portrait) and broadcast (landscape), so your thumbnails will appear at slightly different sizes for all your viewers, depending on the devices they're viewing your site with.
To cater for all eventualities we advise that you upload imagery at about 2560px on it's longest edge and let Fabrik then manage resizing your imagery for the specific devices your users request it from.
We have some further guides on image formats, and how to choose great cover images: