Hello @DamnDramaQueen,
You can set the media thumbnail size from the rtMedia admin settings. You will see different media types as categories under Media Sizes tab.
Thumbnail size reflects the media under media gallery( e.g. under media tab).
Medium size will reflect the activity media posts.
Large size will be used when the media is displayed in the lightbox.
Related document link - https://rtmedia.io/docs/settings/image-sizes/
You can modify the image size according to your requirement and try uploading new media to see the impact.
Let us know if you have any doubts. Also, share some screenshots so that we can get your query quickly.
Thanks.