React Navigation modal height

Here's an example of how to achieve this in react-navigation v3.x:

App Container

const TestRootStack = createStackNavigator(
  {
    TestRoot: TestRootScreen,
    TestModal: {
      screen: TestModalScreen,
      navigationOptions: {
        /**
         * Distance from top to register swipe to dismiss modal gesture. Default (135)
         * https://reactnavigation.org/docs/en/stack-navigator.html#gestureresponsedistance
         */
        gestureResponseDistance: { vertical: 1000 }, // default is 135 },
      },
    },
  },
  {
    headerMode: 'none',
    mode: 'modal',
    transparentCard: true,
  },
);

const AppContainer = createAppContainer(TestRootStack);

Root Screen

class TestRootScreen extends React.Component {
  render() {
    return (
      <SafeAreaView style={styles.container}>
        <Button title="Show Modal" onPress={() => this.props.navigation.navigate('TestModal')} />
      </SafeAreaView>
    );
  }
}

const styles = StyleSheet.create({
  container: {
    flex: 1,
    backgroundColor: 'blue',
    alignItems: 'center',
    justifyContent: 'center',
  },
});

Modal Screen

class TestModalScreen extends React.Component {
  render() {
    return (
      <SafeAreaView style={styles.container}>
        <View style={styles.innerContainer} />
      </SafeAreaView>
    );
  }
}

const styles = StyleSheet.create({
  container: {
    flex: 1,
    backgroundColor: 'transparent',
    justifyContent: 'flex-end',
  },
  innerContainer: {
    position: 'absolute',
    bottom: 0,
    left: 0,
    right: 0,
    top: 100,
    backgroundColor: 'red',
  },
});

In your stacknavigator you can set these options:

  mode: 'modal',
    headerMode: 'none',
    cardStyle:{
      backgroundColor:"transparent",
      opacity:0.99
  }

And in your modal screen:

class ModalScreen extends React.Component {

  render() {
    return (
      <View style={{ flex: 1 ,flexDirection: 'column', justifyContent: 'flex-end'}}>
          <View style={{ height: "50%" ,width: '100%', backgroundColor:"#fff", justifyContent:"center"}}>
            <Text>Testing a modal with transparent background</Text>
          </View>
      </View>
    );
  }
}

Also you can refer to this expo snack https://snack.expo.io/@yannerio/modal that I've created to show how it works, or you could use React Native Modal