Using CB 1.3.1 and using <div> based registration form, I'm aware how we can override the default width (25 visible characters??? ) of the form 'input and select' text fields with our own values. Where can we / Can we update the default value in another file, as this would be much quicker?

I'm not quite sure what you are asking. You can alter the form's css. To find out which parameter/file to change you can use tools like firebug for firefox.