Confused: change user_login-form

I am in the process of creating a theme. Next step is to shorten the size of the two fields on the user-login-form. It's now 60 and I want to change it to 45 (or something). Whatever I try, it's not working.

I've read all this, but am still confused of what to do...

forms_api

http://api.drupal.org/api/file/developer/topics/forms_api.html/6

user_login-page @ api.drupal.org

http://api.drupal.org/api/function/user_login

The theme registry for special cases

http://drupal.org/node/223463

Overriding themable output in the theming guide

http://drupal.org/node/173880

Modifying Forms in Drupal 5 and 6 on the lullabot-site

http://www.lullabot.com/articles/modifying-forms-5-and-6

I found the code I want to override in the user.module.
From what I understand I could just paste the appropriate function in my theme's template.php when using Drupal 5. After changing the function name to themename_user_login() this would work.
In Drupal 6 I have to register the function in template.php and then put the new function somewhere else!?

This is where it goes wrong. What to put exactly in template.php? Where to put the new function?

Is someone out there willing to post a step-by-step procedure, thereby providing answers to my questions?