This method was deprecated in version 1.5 of PlayN, the size of your game view on Android/iOS is dictated by the device. The size on Java is configured via JavaPlatform.Config. The size in HTML is configured by the size of your playn-root div.
For example on the java backend:
public class GameJava {
public static void main(String[] args) {
//Setting the size of the Java Backend
Config config = new Config();
config.width = 800;
config.height = 480;
JavaPlatform platform = JavaPlatform.register(config);
platform.assets().setPathPrefix("your/package/resources");
PlayN.run(new Game());
}
}