viewportSize

The size of the viewport into which the composition is rendering. This could be as lage as the entire screen or as small as an individual view within a larger native screen.

This does not offer any information on the size of the individual composables which are rendering within the composition, but is the frame into which they will render. The root composable if stretching to fill the viewport will match this size.

A null viewport size indicates it has not been resolved yet.