Personally, I opt for making my own comment fields and hiding the default one, as that gives more control over how they work and appear (I have a standard one I use on almost all of my forms that is done with a table, that includes date, user, and notes - along with Javascript that populates a note automatically when the form is submitted).

The built-in comment field is somewhat limited.  You can reduce the maximum character length of the field with some simple Javscript, like this:

$(document).ready(function () {
  $('#comments').attr('maxlength', '50');
});

But you can't use the same code to extend it beyond the default 500 characters - it'll set the attribute, and you can see it set when you inspect element on the browser - but it still limits to 500 characters.