When an input of type text is selected user cannot change its properties from Design mode. 

Workaround: 

<telerik:RadEditor runat="server" ID="RadEditor1">
    <Content>
        <input type="text" style="width: 100px; height: 22px;" />
        <br />
        
        <textarea></textarea>
    </Content>
</telerik:RadEditor>

<script>
    Telerik.Web.UI.RadEditor.prototype.getSelectedElement = function (range) {
        var editor = this;
        var contentArea = editor.get_contentArea();
        var $ = $telerik.$;
        var $Editor = Telerik.Web.UI.Editor;
        var utils = $Editor.Utils;

        if (editor.get_renderMode() === Telerik.Web.UI.RenderMode.Mobile) {
            var highlighted = $(contentArea).find(".__reTextHighlight,.__reNodeHighlight");
            if (highlighted.length) {
                var first = highlighted.get(0);
                var last = highlighted.last().get(0);
                return first === last ? first :
                    utils.findCommonAncestor(first, last);
            }
        }

        range = range || editor.getDomRange();
        if (!range) {
            var activeElement;
            try {
                activeElement = editor.get_document().activeElement;
            } catch (ex) { return undefined; }
            return $(activeElement).is('input[type=text],input[type=password],textarea') ?
                activeElement : undefined;
        }

        var container = range.commonAncestorContainer;
        if (container === contentArea || $.contains(contentArea, container)) {
            var selectedNode = $Editor.RangeEdges.commonNode(range);
            var isTextNode = utils.isTextNode(selectedNode);

            return utils.isTextNode(selectedNode) ? selectedNode.parentNode : selectedNode;
        }
    };
</script>