fillMaxSize

FillHalfSizeModifier

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

SimpleFillModifier

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