When the "Body" field is required, how can I force users to input, say, at least 400 characters?
You should be able to just use the Field validation module to set a minimum length on the body field.


  This module adds an extra tab to each field instance, allowing you to
  specify validation rules for your field instances.
  
  By using the hooks provided by this module, you can also define your
  own validation rules in your own modules.