This is still coding that you would need to do in the template, or code that is showing the images. You should return the image array form the field. Then when you’re code is displaying to either show the full size image or the other image size based on the type of image.
Welcome to the Advanced Custom Fields community forum.
Browse through ideas, snippets of code, questions and answers between fellow ACF users
ACF PRO’s Flexible Content field allows you to create smaller pieces of UI and compose them into whole flexible patterns. In our latest article, we show how to use it to create swappable site sections and integrate it all in a theme.https://t.co/ZRocH8oJSp