Adjust the TextView TextSize immediately for any Android device.

ResizeTextHelper.java

import android.app.AlertDialog;
import android.app.Dialog;
import android.app.DialogFragment;
import android.app.FragmentManager;
import android.content.Context;
import android.content.DialogInterface;
import android.content.res.Resources;
import android.os.Bundle;
import android.util.DisplayMetrics;
import android.util.Log;
import android.util.TypedValue;
import android.view.View;
import android.widget.EditText;
import android.widget.LinearLayout;
import android.widget.SeekBar;
import android.widget.TextView;

/**
 * ResizeTextHelper
 *
 * Adjust the TextView TextSize immediately for any Android device.
 * Mostly for designer use.
 *
 * Copyright (c) 2014 @author extralam @ HongKong (http://ah-lam.com)
 *
 * Permission is hereby granted, free of charge, to any person obtaining a copy
 * of this software and associated documentation files (the "Software"), to deal
 * in the Software without restriction, including without limitation the rights
 * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
 * copies of the Software, and to permit persons to whom the Software is
 * furnished to do so, subject to the following conditions:
 *
 * The above copyright notice and this permission notice shall be included in
 * all copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
 * THE SOFTWARE.
 *
 * Usage:
 * TextView textview = (TextView) fidnViewById(R.id.yourtextview);
 * ResizeTextHelper.attach(getFragmentManager(), textview);
 *
 */
public class ResizeTextHelper {

    /**
     *
     * @param fm    Get the Current FragmentActivity Fragment Manager to generate the DialogFragment
     * @param v     V is the TextView or EditText , you want to adjust the text size.
     */
    public static void attach(FragmentManager fm , View v){
        if(v instanceof TextView){
            v.setOnClickListener(new ViewClick(fm));
        }else{
            return;
        }
    }

    /**
     * private function add on Click Listener
     */
    private static class ViewClick implements View.OnClickListener{
        FragmentManager fm;
        private ViewClick(FragmentManager fm){
            this.fm = fm;
        }
        @Override
        public void onClick(View v) {
            ResizeTextHelper.showResizeTextDialog(fm, (TextView) v);
        }
    }

    /**
     * Create the DialogFragment
     * @param fm
     * @param orgTextView
     */
    private static void showResizeTextDialog(FragmentManager fm , TextView orgTextView) {
        DialogFragment newFragment = new ResizeTextDialog(orgTextView);
        newFragment.show(fm, "showResizeTextDialog");
    }

    /**
     * Main DialogFragment Class
     */
    private static class ResizeTextDialog extends DialogFragment{

        private final int FIXED_MAX_SIZE = 65;
        private final int FIXED_TEXT_SIZE = 5;

        private TextView mOrginalTextView;
        private TextView mText;
        private TextView mTextSizeView;
        private SeekBar mSeekBar;
        private float mOrgTextSize;

        private ResizeTextDialog( TextView view) {
            mOrginalTextView = view;
            mOrgTextSize = mOrginalTextView.getTextSize();
            setCancelable(true);
        }

        @Override
        public Dialog onCreateDialog(Bundle savedInstanceState) {
            AlertDialog.Builder builder = new AlertDialog.Builder(getActivity());
            Log.e("atest","mOrgTextSize L " + mOrgTextSize + "," );
            builder.setTitle("ID: " +
                    getResources().getResourceEntryName(mOrginalTextView.getId()) +
                            " - Text Size Adjustment");
            builder.setView(createView());
            builder.setNegativeButton(android.R.string.cancel, new DialogInterface.OnClickListener() {
                @Override
                public void onClick(DialogInterface dialog, int which) {
                    mOrginalTextView.setTextSize(TypedValue.COMPLEX_UNIT_DIP,
                            convertPixelsToDp(mOrgTextSize, getActivity()));
                    dialog.dismiss();
                }
            });
            builder.setPositiveButton(android.R.string.ok, new DialogInterface.OnClickListener() {
                @Override
                public void onClick(DialogInterface dialog, int which) {
                    dialog.dismiss();
                }
            });
            return builder.create();
        }

        @Override
        public void onCancel(DialogInterface dialog) {
            mOrginalTextView.setTextSize(TypedValue.COMPLEX_UNIT_DIP,
                    convertPixelsToDp(mOrgTextSize, getActivity()));
            super.onCancel(dialog);
        }

        private View createView(){
            LinearLayout mContainer = new LinearLayout(getActivity());
            mContainer.setOrientation(LinearLayout.VERTICAL);
            mTextSizeView = new TextView(getActivity());
            mTextSizeView.setTextSize(TypedValue.COMPLEX_UNIT_DIP, 22);
            mTextSizeView.setText("Size : " + convertPixelsToDp(mOrgTextSize, getActivity()));
            mText = new TextView(getActivity());
            mText.setText(mOrginalTextView.getText());
            mText.setTextSize(TypedValue.COMPLEX_UNIT_DIP, convertPixelsToDp(mOrgTextSize, getActivity()));
            mSeekBar = new SeekBar(getActivity());
            mSeekBar.setMax(FIXED_MAX_SIZE);
            mSeekBar.setProgress((int)convertPixelsToDp(mOrgTextSize , getActivity()) - FIXED_TEXT_SIZE);
            mSeekBar.setOnSeekBarChangeListener(new SeekBar.OnSeekBarChangeListener() {
                @Override
                public void onProgressChanged(SeekBar seekBar, int progress, boolean fromUser) {
                    mTextSizeView.setText("Size : " + (progress + FIXED_TEXT_SIZE) + "dp");
                    mText.setTextSize(TypedValue.COMPLEX_UNIT_DIP, progress + FIXED_TEXT_SIZE);
                    mOrginalTextView.setTextSize(TypedValue.COMPLEX_UNIT_DIP, progress + FIXED_TEXT_SIZE);
                }

                @Override
                public void onStartTrackingTouch(SeekBar seekBar) {

                }

                @Override
                public void onStopTrackingTouch(SeekBar seekBar) {

                }
            });
            mContainer.addView(mText);
            mContainer.addView(mTextSizeView);
            mContainer.addView(mSeekBar);
            return mContainer;
        }

    }

    /**
     * This method converts dp unit to equivalent pixels, depending on device density.
     *
     * @param dp A value in dp (density independent pixels) unit. Which we need to convert into pixels
     * @param context Context to get resources and device specific display metrics
     * @return A float value to represent px equivalent to dp depending on device density
     */
    private static float convertDpToPixel(float dp, Context context){
        Resources resources = context.getResources();
        DisplayMetrics metrics = resources.getDisplayMetrics();
        float px = dp * (metrics.densityDpi / 160f);
        return px;
    }

    /**
     * This method converts device specific pixels to density independent pixels.
     *
     * @param px A value in px (pixels) unit. Which we need to convert into dp
     * @param context Context to get resources and device specific display metrics
     * @return A float value to represent dp equivalent to px value
     */
    private static float convertPixelsToDp(float px, Context context){
        Resources resources = context.getResources();
        DisplayMetrics metrics = resources.getDisplayMetrics();
        float dp = px / (metrics.densityDpi / 160f);
        return dp;
    }
}