CODE EXAMPLE FOR JAVASCRIPT

react native asign width to image

import { Dimensions } from "react-native";
const win = Dimensions.get('window');

<Image
  style={{
    width: win.width/2,
    height: win.width/2,
    resizeMode: "contain",
    alignSelf: "center",
    borderWidth: 1,
    borderRadius: 20,
  }}
  source={{uri:'https://facebook.github.io/react/img/logo_og.png'}}
  resizeMode="stretch"
/>