How to use different styles for android and ios on react-native?

There are many ways to achieve this. The simplest one in your case would be to use Platform.OS:

var {Platform} = React;

var styles = StyleSheet.create({
    height: (Platform.OS === 'ios') ? 200 : 100
});

From https://facebook.github.io/react-native/docs/platform-specific-code, you can use Platform.select

There is also a Platform.select method available, that given an object containing Platform.OS as keys, returns the value for the platform you are currently running on.

import { Platform, StyleSheet } from 'react-native'

const styles = StyleSheet.create({
  container: {
    flex: 1,
    ...Platform.select({
      ios: {
        backgroundColor: 'red',
      },
      android: {
        backgroundColor: 'blue',
      },
    }),
  }
})