Modify size of thumbnail images

Cyrille, the plugin uses default WordPress featured image functionality and does not define the size of featured image.

The featured image or post thumbnail are a WordPress theme feature, this is usually defined by your WordPress theme. If you would like to use a custom size for featured images for your Menu Item Entries, then you would need to either modify your theme’s featured image size, or you can create a new custom image size. However, if you create a custom image size the plugin will not be able to detect it unless you modify the plugin.

Check out How to add Featured Image or Post Thumbnails in WordPress, scroll down to Theme Developer’s section.