<?xml version="1.0" encoding="utf-8"?>

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
              android:layout_width="fill_parent"
              android:layout_height="wrap_content"
              android:padding="8dp"
              android:background="@color/flow_blue"
              android:gravity="center_vertical"
              android:orientation="horizontal">


    <TextView
            android:id="@+id/first"
            android:layout_width="fill_parent"
            android:layout_height="wrap_content"
            android:text=""
            android:textSize="18sp"
            android:textColor="@color/white"
            android:layout_marginLeft="30dp"
            android:textStyle="bold"
            android:gravity="left"/>


</LinearLayout>

<?xml version="1.0" encoding="utf-8"?>

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
              android:layout_width="fill_parent"
              android:layout_height="wrap_content"
              android:padding="8dp"
              android:background="@color/flow_blue"
              android:gravity="center_vertical"
              android:orientation="horizontal">


    <TextView
            android:id="@+id/first"
            android:layout_width="fill_parent"
            android:layout_height="wrap_content"
            android:text=""
            android:textSize="14sp"
            android:textColor="@color/white"
            android:layout_marginLeft="30dp"
            android:textStyle="bold"
            android:gravity="left"/>


</LinearLayout>

Reduce ExpandableListView header text size
Reduce ExpandableListView header text size
