public void OnTabSelected(ActionBar.Tab tab, FragmentTransaction ft)
{
        DisplayContactFragment(tab);
}

private void DisplayContactFragment(ActionBar.Tab tab)
{
        ContactFragment fragment = new ContactFragment();
        SupportFragmentManager.BeginTransaction().Replace(Resource.Id.fragmentContainer, fragment).Commit();
}

-------------------------------------

    public class ContactFragment : Fragment
    {
        public override View OnCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState)
        {
            View v = inflater.Inflate(Resource.Layout.Contact, container, false);
            v.LayoutParameters = new ViewGroup.LayoutParams(new ViewGroup.LayoutParams(
                ViewGroup.LayoutParams.FillParent, ViewGroup.LayoutParams.FillParent));
    }

-------------------------------------

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="vertical" android:layout_width="fill_parent"
  android:fillViewport="true"
    android:layout_height="fill_parent">
  <LinearLayout android:background="#FF0000"
        android:layout_height="0dp" android:layout_width="fill_parent"
        android:layout_weight="1" />
  <LinearLayout android:background="#00FF00"
        android:layout_height="0dp" android:layout_width="fill_parent"
        android:layout_weight="1" />
</LinearLayout>