Its always nice when the content in a callout is relatively similar in character length, however, sometimes it can be hard to ensure that. Interactive callouts like flip-boxes and slide-boxes can help solve this problem. Just be careful to use these as accents with links to read more on another page – never put too much content in these.
Callouts are a good way to illustrate the different features a site has, sometimes with the option to learn more about these features. Create a simple text callout or add an image, an icon or a call-to-action link to it.