According to their docs Chromecast only supports 1080x720 images, which I think does not apply to the New Chromecast that supports 1080p. As a person who closely follows documentation I am unsure if I am using the wrong one (for the old chromecast), a outdated one or if this still applies to the new chromecast?
Currently, receiver apps will see 720p for images but please make sure your app is responsive and doing the right sizing so that if this changes in future, your app can adjust automatically.