The problem is, your template can contain several HTML elements, so MVC won't know to which one to apply your size/class. You'll have to define it yourself.
Make your template derive from your own class called TextBoxViewModel:
public class TextBoxViewModel
{
  public string Value { get; set; }
  IDictionary<string, object> moreAttributes;
  public TextBoxViewModel(string value, IDictionary<string, object> moreAttributes)
  {
    // set class properties here
  }
  public string GetAttributesString()
  {
     return string.Join(" ", moreAttributes.Select(x => x.Key + "='" + x.Value + "'").ToArray()); // don't forget to encode
  }
}
In the template you can do this:
<input value="<%= Model.Value %>" <%= Model.GetAttributesString() %> />
In your view you do:
<%= Html.EditorFor(x => x.StringValue) %>
or
<%= Html.EditorFor(x => new TextBoxViewModel(x.StringValue, new IDictionary<string, object> { {'class', 'myclass'}, {'size', 15}}) %>
The first form will render default template for string. The second form will render the custom template.
Alternative syntax use fluent interface:
public class TextBoxViewModel
{
  public string Value { get; set; }
  IDictionary<string, object> moreAttributes;
  public TextBoxViewModel(string value, IDictionary<string, object> moreAttributes)
  {
    // set class properties here
    moreAttributes = new Dictionary<string, object>();
  }
  public TextBoxViewModel Attr(string name, object value)
  {
     moreAttributes[name] = value;
     return this;
  }
}
   // and in the view
   <%= Html.EditorFor(x => new TextBoxViewModel(x.StringValue).Attr("class", "myclass").Attr("size", 15) %>
Notice that instead of doing this in the view, you may also do this in controller, or much better in the ViewModel:
public ActionResult Action()
{
  // now you can Html.EditorFor(x => x.StringValue) and it will pick attributes
  return View(new { StringValue = new TextBoxViewModel(x.StringValue).Attr("class", "myclass").Attr("size", 15) });
}
Also notice that you can make base TemplateViewModel class - a common ground for all your view templates - which will contain basic support for attributes/etc.
But in general I think MVC v2 needs a better solution. It's still Beta - go ask for it ;-)