Just edit your page and click on the image and image option will pop out click on edit option then you will see the image size option under display settings choose full size and update your page.
Which version of WordPress you are using? If you are using 5.0+ you have to add the Image Block and you will get block settings along with image size option on the right sidebar of the page editor.
If you are using the WP version below 5.0 and adding the image via code you will have to add some class to make the image full size.