Google Discover
Short answer
Indexed content that meets Discover's content policies is automatically eligible for Discover, with no special markup, though eligibility is no guarantee. Google recommends large images at least 1200px wide and titles without clickbait. Discover traffic is less predictable than search traffic and should be treated as supplemental.
How content gets in
Section titled “How content gets in”Images
Section titled “Images”max-image-preview:large is a robots meta value. See Snippet controls for AI Overviews and AI Mode.
Traffic changes
Section titled “Traffic changes”The Performance report for Discover in Search Console shows impressions, clicks and CTR for the last 16 months, once a minimum impression threshold is reached.
Generative AI features in Discover
Section titled “Generative AI features in Discover”See Search generative AI control.
This site
Section titled “This site”Indexable pages here set max-image-preview:large. Each entry has its own share image showing its title, type and evidence level, rather than one site-wide image. The images are 1200×630, the common Open Graph size. That is about 1.91:1, not the 16:9 Google recommends for Discover.