Ancho 100% reaccionó el nativo
import { Dimensions } from 'react-native';
width: ${(Dimensions.get('window').width)}px;
Marton
import { Dimensions } from 'react-native';
width: ${(Dimensions.get('window').width)}px;<View style={{backgroundColor: '#000000', alignSelf: 'flex-start' }}>
<Text style={{color: '#ffffff'}}>
Sample text here
</Text>
</View>