After adding images to an article; you can adjust them via the source code view.

The size value within the article-image element allows you to either resize the image or remove the value all together in order to have the image show up at its original size.

Standard image size

<article-image uid="65391.custom.4.94ef2168-6062-4c7f-a659-7ddcc033151e.jpg" contenteditable="false" size="600"></article-image>

Other sizes using size= that can be used; include: 300, 600, 900 & 1200

For the image to show at original size

<article-image uid="65391.custom.4.94ef2168-6062-4c7f-a659-7ddcc033151e.jpg" contenteditable="false"></article-image>

Generally it's better to just remove the size value as this will prevent the image in it's original size, and will be more presentable.