Use LinearyLayout to wrap your ImageView and another invisible View, and set layout_weight both to 0.5, then the ImageView should be half the size.
Here is an example for you showing how to set the image half the screen size
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:orientation="vertical"
android:layout_width="match_parent"
android:layout_height="match_parent">
<RelativeLayout
android:layout_width="match_parent"
android:layout_height="0dp"
android:layout_weight="0.5"
android:background="#ff0000"
>
<ImageView
android:layout_width="match_parent"
android:layout_height="match_parent"
android:src="https://stackoverflow.com/questions/14544327/@drawable/your_image"
android:adjustViewBounds="true"
/>
</RelativeLayout>
<View
android:layout_width="match_parent"
android:layout_height="0dp"
android:layout_weight="0.5"
android:background="#00ff00"
/>
</LinearLayout>
And this is the result
If you want to put your image in center of the screen, you can use the same idea and set proper weight to achieve your goal. Good luck!