The Code Box is simply a textarea field that automatically allows you to enter HTML code. It's identical to the Paragraph Text field except that it automatically allows HTML code. The Paragraph Text field by default will convert any HTML code to their appropriate HTML entity. However, since the Paragraph Text field now has an option to allow HTML code, you could simply use that field type and enable HTML code.
There are a few pre-defined sizes for the field. The sizes are relative to the size of your browser viewport.