Specific style for TinyMCE in ListProperty

Hi,

I have a Page: 

[ContentType(
 GUID = "86f7b89e-1c56-40fa-a808-44e8e1e262c2",
 DisplayName = "Columns with text blobs",
 Description = "Text blobs with heading and body, organized in columns")]
 public class TextBlobsInColumnsBlock : BlockData
 {
 [CultureSpecific]
 [Display(Name = "Add margin in top of element", Order = 10)]
 public virtual bool AddTopMargin { get; set; }

[Required]
 [CultureSpecific]
 [Display(Name = "Headline", Order = 20)]
 public virtual string Headline { get; set; }

[Display(
 Name = "Text blobs",
 Description = "Add text blob",
 Order = 30)]
 [EditorDescriptor(EditorDescriptorType = typeof(CollectionEditorDescriptor))]
 public virtual IList TextBlobs { get; set; }

}

With a ListProperty list of :

public class TextBlob:BlockData
 {
 public string Heading { get; set; }

public XhtmlString Body { get; set; }

}
}

When adding or editing a text blob the TinyMce is rendered too wide like this:

To make the TineMce slimmer I added this class:

[ModuleDependency(typeof(TinyMceInitialization))]
public class TextBlobMceSettings : IConfigurableModule
{
public void Initialize(InitializationEngine context)
{
}

public void Uninitialize(InitializationEngine context)
{
}

public void ConfigureContainer(ServiceConfigurationContext context)
{


context.Services.Configure(config =>
{
config.For(x => x.Body).Width(460);
});
}
}

This approach works for normal page properties. But not in this senario. 

Any ideas to make this work? Or is there another way to do this?