Using the formatting menu, you can set the the image to full page width or content width. With content width selected, you can also click and drag the bottom-right corner of the image for a custom size.
Select the image on the form, and you'll see a range of formatting options above the image. Select the first option (full page width), and the image will take up the full width of the page.
The second option in the menu can also be used to make the image the same width as the rest of the form content (content width).
If you want to manually make the image smaller or larger, first ensure the image is set to content width. Then, you can grab the corner handle on the bottom-right of the image, and drag it to resize.
If you've used the built-in image editor, you may have noticed a Resize option. This can be used to change the pixel size of the original image, but it does not control how this image is displayed on your form page.
To change how the image appears in your layout, stick to the methods outlined above. The Resize tool in the image editor is better used for shrinking unnecessarily large image files down to a more appropriate size, for the sake optimizing page load times.