Set its layout_height="0dp"*, add a blank View beneath it (or blank ImageView or just a FrameLayout) with a layout_height also equal to 0dp, and set both Views to have a layout_weight="1"
This will stretch each View equally as it fills the screen. Since both have the same weight, each will take 50% of the screen.
*See adamp’s comment for why that works and other really helpful tidbits.