Hi there,
I believe the Post Image options are for the images on the blog page, not for the blog entry itself.
If you want to re-size the image in the blog entry, WordPress has an image scaling option in the media library you can access by clicking “Edit image” when selecting the image you want to use. More info here: https://en.support.wordpress.com/images/image-editing/
To change the position, you can try adding the following CSS:
.page-header-image-single {
text-align: center;
}
Adding CSS: https://generatepress.com/knowledgebase/adding-css/
Let me know if this helps 🙂