TypeConverter

    public class Class1 : Component
    {
        private Size _size;

        public Class1()
        {
            _size = new Size();
        }

     [DesignerSerializationVisibility(DesignerSerializationVisibility.Content)]
        public Size Size
      {...
      }

  public class SizeConverter : TypeConverter       
    {
     ...
     }
  [TypeConverter(typeof(SizeConverter))] //    
    public class Size
   {
   }