Whether or not you’re using gutenberg, selecting the image size to be shown/returned is not a feature built into ACF image fields. See the documentation https://www.advancedcustomfields.com/resources/image/
You can return the image ID, the URL which always returns the original url, or an image array that contains information on various sizes. If you want to be able to select the site then you’d need to create some type of choice field to allow selection and then deal with showing the correct size image in your code.
