module.config

<?xml version="1.0" encoding="utf-8"?>
<module>
    <assemblies>
	    <!-- This adds the template assembly to the "default module" -->
        <add assembly="EpiSandbox" />
    </assemblies>
    <clientResources>
        <add name="epi-cms.widgets.base" path="Styles/Styles.css" resourceType="Style"/>
    </clientResources>
    <dojo>
        <!-- This maps "shell" to ~/ClientResources/Scripts for the dojo loader configuration -->
        <paths>
            <add name="shell" path="Scripts" />
        </paths>
    </dojo>
</module>

PreviewableTextArea.js

define("shell/editors/PreviewableTextArea", [
    "dojo/_base/array",
    "dojo/_base/declare",
    "dojo/dom-construct",

    "epi/shell/widget/ValidationTextarea", 	// added to change to ValidationTextarea from ValidationTextBox
    "epi-cms/contentediting/editors/_PreviewableEditor"
], function (
    array,
    declare,
    domConstruct,

    TextArea,  // added to change to ValidationTextarea from ValidationTextBox
    _PreviewableEditor
) {
    return declare([_PreviewableEditor], {
        // tags:
        //      internal

        required: false,

        // controlParams: Array
        //      Properties to copy from this widget into the wrapped control
        controlParams: ["regExp", "required", "invalidMessage"],
	    
        // this function creates the TextArea to override the TextBox that would otherwise display
        buildRendering: function () {
            this.control = new TextArea();
            this.inherited(arguments);
        },

        _getRegExpAttr: function () {
            return this.regExp || "";
        }
    });
});

PreviewableTextareaDescriptor.cs

using EPiServer.Cms.Shell.UI.ObjectEditing;
using EPiServer.Shell.ObjectEditing;
using EPiServer.Shell.ObjectEditing.EditorDescriptors;
using System;
using System.Collections.Generic;

namespace EpiSandbox.Business.EditorDescriptors
{
    [EditorDescriptorRegistration(TargetType = typeof(string), UIHint = "previewtextarea")]
    public class PreviewableTextareaDescriptor : EditorDescriptor
    {
        public PreviewableTextareaDescriptor()
        {
            base.ClientEditingClass = "shell/editors/PreviewableTextArea";
        }

        public override void ModifyMetadata(ExtendedMetadata metadata, IEnumerable<Attribute> attributes)
        {
            base.ModifyMetadata(metadata, attributes);
            if (metadata is ContentDataMetadata)
            {
                metadata.EditorConfiguration["style"] = "width:400px;";
            }
        }
    }
}