WP does not differentiate what size image is being set as the post thumbnail/featured image. All images sizes for any given image all have the same ID value, each image size does not get its own ID. Because of this it is not possible to insert a specific sized image as the featured image.
What size image is shown on the site is dependent on how the image is added to the theme. There are several functions that can be used to add an image to a page, for example: https://developer.wordpress.org/reference/functions/the_post_thumbnail/
the_post_thumbnail($size);
If size is not given in this function then it is assumed you want to show the “post-thumbnail” size, whatever that is.
Your theme code can be adding the image in any number of ways, but it is there that you need to adjust the image size that is displayed and not when adding an image as the featured image for a post.
