Layout

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools" android:layout_width="match_parent"
    android:layout_height="match_parent" android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    android:paddingBottom="@dimen/activity_vertical_margin" tools:context=".MainActivity">

    <LinearLayout
        android:orientation="horizontal"
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:layout_alignParentTop="true"
        android:layout_alignParentLeft="true"
        android:layout_alignParentStart="true"
        android:id="@+id/linearLayout">

        <Spinner
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:id="@+id/spinner"
            android:entries="@array/shipping_companies"/>

        <EditText
            android:layout_width="0dp"
            android:layout_height="match_parent"
            android:id="@+id/editText"
            android:layout_weight="1" />
    </LinearLayout>

    <Button
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="New Button"
        android:id="@+id/testingButton"
        android:layout_centerVertical="true"
        android:layout_centerHorizontal="true" />
</RelativeLayout>

Strings (Array)

<resources>
    <string name="app_name">Package Tracker</string>

    <string name="hello_world">Hello world!</string>
    <string name="action_settings">Settings</string>

    <array name="shipping_companies">
        <item>USPS</item>
        <item>FedEx</item>
        <item>UPS</item>
    </array>
</resources>

Try this:

`

    <Spinner
        android:layout_width="wrap_content"
        android:layout_height="match_parent"
        android:id="@+id/spinner"
        android:entries="@array/shipping_companies"/>

    <EditText
        android:layout_width="0dp"
        android:layout_height="match_parent"
        android:id="@+id/editText"
        android:layout_weight="1" />
</LinearLayout>`