fillMaxWidth

FillHalfWidthModifier

@Composable
fun FillHalfWidthModifier() {
    Box(Modifier.requiredSize(100.dp).background(Color.Red), contentAlignment = Alignment.Center) {
        // The inner Box will be (50.dp x 30.dp).
        Box(
            Modifier.fillMaxWidth(fraction = 0.5f)
                .requiredHeight(30.dp)
                .background(color = Color.Magenta)
        )
    }
}

SimpleFillWidthModifier

@Composable
fun SimpleFillWidthModifier() {
    Box(Modifier.fillMaxWidth().background(Color.Red), contentAlignment = Alignment.Center) {
        Box(Modifier.size(100.dp).background(color = Color.Magenta))
    }
}