When a groupbox is inserted and start typing text, the fieldset stays fixed in size.

You can use the following resolved command:

<telerik:RadEditor runat="server" ID="RadEditor1">
</telerik:RadEditor>

<script type="text/javascript">
	Telerik.Web.UI.Editor.CommandList.InsertGroupbox = function myfunction(commandName, editor, args) {
		var localizedTitle = editor.getLocalizedString("RadEditorGroupboxTitle", "Title");
		var localizedContent = editor.getLocalizedString("RadEditorGroupboxContent", "Content...");
		value = "<fieldset style='width: 200px; min-height: 76px'> <legend>" + localizedTitle + "</legend>" + localizedContent + " </fieldset> ";

		editor.pasteHtml(value, commandName);
	}
</script>