How to use Modifier.contentSize in Jetpack Compose?

I think you are talking about the animateContentSize function.

Here I have an example that might help

Column(
        modifier = Modifier
            .fillMaxWidth()
            .padding(16.dp)
            // This `Column` animates its size when its content changes.
            .animateContentSize()
    ) {
        Row {
            Icon(
                imageVector = Icons.Default.Info,
                contentDescription = null
            )
            // ...
        }
        if (expanded) { // If the expanded value is changed then the animateContentSize will be triggered
            Spacer(modifier = Modifier.height(8.dp))
            Text(
                text = stringResource(R.string.lorem_ipsum),
                textAlign = TextAlign.Justify
            )
        }
    }

From the Jetpack Compose Animation codelab: