Can I design a customized size activity in Android Studio 3.0?

Normally, a activity will display a maximum screen. I hope to display a window with width 400px and height 500px, but the following is still display a full window, why?

MainActivity

class MainActivity : AppCompatActivity() {

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
    }
}

activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.ConstraintLayout
        xmlns:android="http://schemas.android.com/apk/res/android"
        xmlns:tools="http://schemas.android.com/tools"
        xmlns:app="http://schemas.android.com/apk/res-auto"
        android:layout_width="400px"
        android:layout_height="500px"
        tools:context=".MainActivity">

    <TextView
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="Hello World OK!"
            app:layout_constraintBottom_toBottomOf="parent"
            app:layout_constraintLeft_toLeftOf="parent"
            app:layout_constraintRight_toRightOf="parent"
            app:layout_constraintTop_toTopOf="parent"/>

</android.support.constraint.ConstraintLayout>

To KeLiuyue

Thanks! but your code doesn't work, the window is full screen after run your code.

class MainActivity : AppCompatActivity() {

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)

        val m = windowManager
        val d = m.defaultDisplay
        val p = window.attributes
        p.height = dp2px(this, 500f)
        p.width = dp2px(this, 400f)
        window.attributes = p

    }

    private fun dp2px(context: Context, dp: Float): Int {
        val scale = context.getResources().getDisplayMetrics().density
        return (dp * scale + 0.5f).toInt()
    }

}

from Can I design a customized size activity in Android Studio 3.0?