In the Featured Image settings you have:
1. Media Attachment Size – this allows you choose the actual image to be loaded
2. Set the Width and Height – this allows you to change the size of the image being displayed. Which is handy if you have an want to align the image to one side and other styles.
If you want to load the exact image then choose the correct Media Attachment Size and leave the Width and Height empty.
This article explains how to add new Media Attachment sizes for your choosing:
https://docs.generatepress.com/article/adding-image-sizes/
Make sure to run the Regenerate Thumbnails plugin to update any existing images. Any new images you upload will automatically create them.