Required XML attribute "adSize" was missing?

1) Use the XML namespace  "http://schemas.android.com/apk/res-auto". 

Using XML

<com.google.android.gms.ads.AdView



    xmlns:ads="http://schemas.android.com/apk/res-auto"
    android:id="@+id/adView"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    ads:adUnitId="AD_UNIT_ID"
    ads:adSize="BANNER"/>

Programatically

AdView adView = new AdView(getActivity());
adView.setAdUnitId("YourId");
adView.setAdSize(AdSize.BANNER);

final LinearLayout adLinLay = (LinearLayout) view.findViewById(R.id.linear);
adLinLay.addView(adView);

final AdRequest.Builder adReq = new AdRequest.Builder();
final AdRequest adRequest = adReq.build();
adView.loadAd(adRequest);