When adding the adView widget to an Android App in Android Studio, you might initially get the error: “Required xml attribute adsize was missing in android studio” displayed in red on your widget in the design view.
Adding the following line to your activity_main.xml could help solve this problem.
<com.google.android.gms.ads.AdView
android:id="@+id/adView"
android:layout_width="match_parent"
android:layout_height="wrap_content"
xmlns:ads="http://schemas.android.com/apk/res-auto"
ads:adSize="BANNER"
android:layout_weight="1" />