Automatically Scales Sprite Texture2Ds, allowing you to multiply the image size safely, and effectively. Not guaranteed to work on every Sprite, or at all for that matter :P.

SpriteResizer.cs

using System;
using System.Collections.Generic;
using UnityEditor;
using UnityEngine;

/* Unity Sprite Resizer v1.6 © Hotrian 2017
 * 
 * This has only been tested on PNG files.
 * 
 * Usage instructions:
 *  1. Put SpriteResizer.cs at
 *       ..\Assets\Editor\SpriteResizer.cs
 *  2. After Unity recompiles, at the top Click Window > SpriteResizer
 *  3. In the SpriteResizer window, select your Sprite's Texture2D
 *  4. Select a Multiplier value
 *  5. Click Upscale Texture
 *  
 *  This generates replacements for your Sprite, but cannot install them automatically as they are locked by Unity.
 *  Your replacement files will be named:
 *      [originalName.png2]
 *      [originalName.png.meta2]
 *  
 *  To install your new sprite:
 *  1. First, make sure Unity is closed so it is not locking your assets
 *  2. Navigate to your Sprite in File Explorer
 *  3. Delete the [originalName.png] and [originalName.png.meta] files
 *  4. Rename the new files by removing the 2 that has been added after the extension:
 *      [originalName.png2]      -> [originanName.png]
 *      [originalName.png.meta2] -> [originanName.png.meta]
 *  5. Restart Unity and Marvel! 
 * 
 */

public class SpriteResizer : EditorWindow
{
    private Texture2D _tex;
    private Powers _multiplier = Powers.Two;

    [MenuItem("Window/SpriteResizer")]
    public static void ShowWindow()
    {
        EditorWindow.GetWindow(typeof(SpriteResizer));
    }

    void OnGUI()
    {
        _tex = (Texture2D)EditorGUILayout.ObjectField("Sprite", _tex, typeof(Texture2D), false);
        if (_tex == null) return;

        var w1 = _tex.width;
        var w2 = _tex.width * (int)_multiplier;
        var h1 = _tex.height;
        var h2 = _tex.height * (int)_multiplier;
        _multiplier = (Powers)EditorGUILayout.EnumPopup("Multiplier:", _multiplier);
        GUILayout.Label(string.Format("Width:\t({0})\t{1}\nHeight:\t({2})\t{3}", w1, w2, h1, h2), EditorStyles.boldLabel);

        if (GUILayout.Button("Upscale Texture"))
        {
            // Generate some information about this action
            var path = AssetDatabase.GetAssetPath(_tex);
            var filePath = Application.dataPath + path.Substring(6);
            var metaPath = Application.dataPath + path.Substring(6) + ".meta";
            var file = path.LastIndexOf(System.IO.Path.DirectorySeparatorChar);
            if (file == -1) file = path.LastIndexOf(System.IO.Path.AltDirectorySeparatorChar);
            if (file == -1) Debug.LogWarning("Couldn't find last Path Seperator");
            var fileName = path.Substring(file + 1);

            try
            {
                _tex.GetPixels();
            }
            catch (UnityException e)
            {
                if (e.Message.Contains("' is not readable, the texture memory can not be accessed from scripts."))
                {
                    Debug.LogWarning("Texture [" + fileName + "] is not readable.. Doing this the hard way..");
                    _tex = LoadPng(filePath);
                }
                else
                {
                    Debug.LogException(e);
                    return;
                }
            }

            // Attempt to save the resized texture to disk
            Debug.LogWarning("Resizing Texture [" + fileName + "]");
            System.IO.File.WriteAllBytes(filePath + "2", Resize(_tex, (int)_multiplier).EncodeToPNG());
            Debug.LogWarning("Resized. Fixing Sprite Meta...");

            // Parse the Metadata file
            var lines = System.IO.File.ReadAllLines(metaPath);
            var newLines = new List<string>();
            foreach (var line in lines)
            {
                // Handles resizing the Pixels Per Unit
                if (line.TrimStart().StartsWith("spritePixelsToUnits: "))
                {
                    var len = line.IndexOf("spritePixelsToUnits: ", StringComparison.CurrentCulture);
                    var baseVal = int.Parse(line.Substring(len + 21).Trim());
                    switch (baseVal)
                    {
                        case 1:
                        case 4:
                        case 8:
                        case 16:
                        case 32:
                        case 64:
                        case 128:
                        case 256:
                        case 512:
                        case 1024:
                            break;
                        default:
                            Debug.LogWarning("Pixels Per Unit should be a Power of 2! Current value: " + baseVal);
                            break;
                    }
                    var val = baseVal * (int)_multiplier;
                    newLines.Add(line.Substring(0, len + 21) + val);
                } // Handles resizing sliced Sprite X
                else if (line.TrimStart().StartsWith("x: "))
                {
                    var len = line.IndexOf("x: ", StringComparison.CurrentCulture);
                    var val = int.Parse(line.Substring(len + 3).Trim()) * (int)_multiplier;
                    newLines.Add(line.Substring(0, len + 3) + val);
                } // Handles resizing sliced Sprite Y
                else if (line.TrimStart().StartsWith("y: "))
                {
                    var len = line.IndexOf("y: ", StringComparison.CurrentCulture);
                    var val = int.Parse(line.Substring(len + 3).Trim()) * (int)_multiplier;
                    newLines.Add(line.Substring(0, len + 3) + val);
                } // Handles resizing sliced Sprite Width
                else if (line.TrimStart().StartsWith("width: "))
                {
                    var len = line.IndexOf("width: ", StringComparison.CurrentCulture);
                    var val = int.Parse(line.Substring(len + 7).Trim()) * (int)_multiplier;
                    newLines.Add(line.Substring(0, len + 7) + val);
                } // Handles resizing sliced Sprite Height
                else if (line.TrimStart().StartsWith("height: "))
                {
                    var len = line.IndexOf("height: ", StringComparison.CurrentCulture);
                    var val = int.Parse(line.Substring(len + 8).Trim()) * (int)_multiplier;
                    newLines.Add(line.Substring(0, len + 8) + val);
                } // Add all other lines verbatim
                else newLines.Add(line);
            }

            // Attempt to save the new metadata to disk 
            try
            {
                System.IO.File.WriteAllLines(metaPath + "2", newLines.ToArray());
            }
            catch (Exception e)
            {
                Debug.LogException(e);
                return;
            }

            Debug.Log("Texture Resized Successfully!");
            Debug.Log(string.Format("Please close Unity, then delete [{0}] and [{1}], and rename [{2}] and [{3}] to replace them.", fileName, fileName + ".meta", fileName + "2", fileName + ".meta2"));
        }
    }

    public static Texture2D LoadPng(string filePath)
    {
        if (!System.IO.File.Exists(filePath)) return null;
        var fileData = System.IO.File.ReadAllBytes(filePath);
        var tex = new Texture2D(2, 2) {filterMode = FilterMode.Point};
        tex.LoadImage(fileData); //..this will auto-resize the texture dimensions.
        return tex;
    }

    /// <summary>
    /// Scale a given texture to a larger multiple of the same texture
    /// </summary>
    Texture2D Resize(Texture2D t, int scale)
    {
        // Generate a Point Filtered texture which is [scale] times larger than the source Texture2D
        var tex = new Texture2D(t.width * scale, t.height * scale, t.format, false) { filterMode = FilterMode.Point };
        var c = t.GetPixels(); // Get the pixel data for the source texture
        var cExpand = new Color[c.Length * scale * scale]; // Create an array to store our working data

        var i = 0;
        var j = 0;

        // Run through every line of the source texture
        for (var h = 0; h < t.height; h++)
        {
            var lJ = j;
            // Repeat this line [scale] number of times
            for (var y = 0; y < scale; y++)
            {
                j = lJ;
                // Run through every column of the source texture
                for (var w = 0; w < t.width; w++)
                {
                    // Repeat this line [scale] number of times
                    for (var x = 0; x < scale; x++)
                    {
                        // Save this color to our working data
                        cExpand[i] = c[j];
                        i++;
                    }
                    j++;
                }
            }
        }

        // Update our new texture with the working data
        tex.SetPixels(cExpand);
        tex.filterMode = FilterMode.Point;
        tex.Apply();
        return tex;
    }

    public enum Powers
    {
        One = 1, // Effectively does nothing. Duplicates the Texture as is.
        Two = 2, // Doubles the Texture size on each axis.
        Four = 4, // Quadruples the Texture size on each axis.
        Eight = 8, // etc.
        Sixteen = 16,
        Thirtytwo = 32,
    }
}