public class misson4 extends AppCompatActivity {

    EditText edt_message;
    TextView txt_size;

    @Override
    protected void onCreate(@Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_mission4);

        edt_message = findViewById(R.id.edt_message);
        txt_size = findViewById(R.id.txt_size);

        Button sendButton = findViewById(R.id.btn_send);
        sendButton.setOnClickListener(new View.OnClickListener() {
            public void onClick(View v) {
                String message = edt_message.getText().toString(); // editText메세지 받아오기
                Toast.makeText(getApplicationContext(), "전송할 메시지\\n\\n" + message, Toast.LENGTH_LONG).show(); // 토스트 표시
            }
        });

        Button closeButton = findViewById(R.id.btn_close);
        closeButton.setOnClickListener(new View.OnClickListener() {
            public void onClick(View v) {
                finish(); // 액티비티 끝내기
            }
        });

        TextWatcher watcher = new TextWatcher() { // 변화 감지
            public void onTextChanged(CharSequence str, int start, int before, int count) { // 텍스타가 바뀌면
                byte[] bytes = null;
                try {
                    bytes = str.toString().getBytes("KSC5601");
                    int strCount = bytes.length; // 바이트 길이
                    txt_size.setText(strCount + " / 80바이트"); // Set textView
                } catch(UnsupportedEncodingException ex) {
                    ex.printStackTrace();
                }
            }

            public void beforeTextChanged(CharSequence s, int start, int count, int after) {

            }

            public void afterTextChanged(Editable strEditable) {
                String str = strEditable.toString();
                try {
                    byte[] strBytes = str.getBytes("KSC5601");
                    if(strBytes.length > 80) {
                        strEditable.delete(strEditable.length()-2, strEditable.length()-1);
                    }
                } catch(Exception ex) {
                    ex.printStackTrace();
                }
            }
        };

        edt_message.addTextChangedListener(watcher); // editText에 watcher추가
    }
}

TextWatcher쪽을 공부할 필요가 있을듯.

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="<http://schemas.android.com/apk/res/android>"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical">

    <EditText
        android:id="@+id/edt_message"
        android:layout_width="match_parent"
        android:layout_height="400dp"
        android:layout_margin="10dp"
        android:singleLine="false"
        android:maxLength="80"
        android:gravity="top"
        android:textSize="36sp"
        android:hint="입력상자"
        android:inputType="textPersonName" />

    <TextView
        android:id="@+id/txt_size"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="0/80 바이트"
        android:gravity="right"
        android:layout_marginRight="10dp"/>

    <LinearLayout
        android:layout_gravity="center"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:orientation="horizontal">

        <Button
            android:id="@+id/btn_send"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="전송"/>
        <Button
            android:id="@+id/btn_close"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="닫기"/>
    </LinearLayout>

</LinearLayout>