<me.grantland.widget.AutofitTextView
   android:id="@+id/output_autofit"
   android:layout_width="match_parent"
   android:layout_height="wrap_content"
   android:text="@string/example"
   android:textSize="50sp"
   android:gravity="center"
   android:singleLine="true"
   autofit:minTextSize="8sp"
   />

<?xml version="1.0" encoding="utf-8"?>
<ScrollView xmlns:android="http://schemas.android.com/apk/res/android"   xmlns:autofit="http://schemas.android.com/apk/res-auto"
 android:layout_width="match_parent"
 android:layout_height="match_parent">
 <LinearLayout
  android:layout_width="match_parent"
  android:layout_height="wrap_content"
  android:orientation="vertical"
  >
  <EditText
   android:id="@+id/input"
   android:layout_width="match_parent"
   android:layout_height="wrap_content"
   android:singleLine="true"
   android:hint="@string/input_hint"
   android:text="@string/example"/>
  <TextView
   android:layout_width="match_parent"
   android:layout_height="wrap_content"
   android:text="@string/label_normal"
   />
  <TextView
   android:id="@+id/output"
   android:layout_width="match_parent"
   android:layout_height="wrap_content"
   android:text="@string/example"
   android:textSize="50sp"
   android:gravity="center"
   />
  <TextView
   android:layout_width="match_parent"
   android:layout_height="wrap_content"
   android:text="@string/label_autofit"
   />
  <me.grantland.widget.AutofitTextView
   android:id="@+id/output_autofit"
   android:layout_width="match_parent"
   android:layout_height="wrap_content"
   android:text="@string/example"
   android:textSize="50sp"
   android:gravity="center"
   android:singleLine="true"
   autofit:minTextSize="8sp"
   />
 </LinearLayout>
</ScrollView>

activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<resources>
 <string name="app_name">Texttest</string>
 <string name="action_settings">Settings</string>
 <string name="hello_world">Hello world!</string>
 <string name="input_hint">text</string>
 <string name="label_normal">Normal:</string>
 <string name="label_autofit">Autofit:</string>
 <string name="example">This is an example</string>
</resources>

package com.example.texttest;
import android.app.Activity;
import android.os.Bundle;
import android.text.Editable;
import android.text.TextWatcher;
import android.view.Menu;
import android.widget.EditText;
import android.widget.TextView;
public class MainActivity extends Activity {
 private TextView mOutput;
 private TextView mAutofitOutput;
 @Override
 protected void onCreate(Bundle savedInstanceState) {
  super.onCreate(savedInstanceState);
  setContentView(R.layout.activity_main);
  mOutput = (TextView)findViewById(R.id.output);
  mAutofitOutput = (TextView)findViewById(R.id.output_autofit);
  ((EditText)findViewById(R.id.input)).addTextChangedListener(new TextWatcher() {
   @Override
   public void beforeTextChanged(CharSequence charSequence, int i, int i2, int i3) {
    // do nothing
   }
   @Override
   public void onTextChanged(CharSequence charSequence, int i, int i2, int i3) {
    mOutput.setText(charSequence);
    mAutofitOutput.setText(charSequence);
   }
   @Override
   public void afterTextChanged(Editable editable) {
    // do nothing
   }
  });
 }
 @Override
 public boolean onCreateOptionsMenu(Menu menu) {
  // Inflate the menu; this adds items to the action bar if it is present.
  getMenuInflater().inflate(R.menu.main, menu);
  return true;
 }
}

MainActivity.java