Ok... you are about to enter one of the most exciting and complex features of Drupal: the form API or - for brevity - FAPI. Some theory first, and then the solution! :)

All forms in Drupal are built by the drupal_get_form() function, that accepts an array as parameter. Each field in the array is basically a field of your form, and each field has a number of proprieties, each of them define additional characteristics of the the field, like for example its default value, if it is required or optional and - yes - in the case of textfields... how large they have to be! You can find a detailed explanation of the structure of form arrays here on the drupal site.

The beauty of the form API is that the function that renders the form invokes a number of hooks at various moments during its building process, so you can implement these hooks in order to "alter" a form before it is finalised and sent to the browser.

The most commonly hooks for form alteration are hook_form_alter() and hook_form_FORM_ID_alter(). The first is executed for any form processed by the drupal engine, the latter only for the specific form named "FORM_ID". I will not get into any more details on the internal working of the form API, but here you can read more.

As for your specific case, I assume you are using the standard "user block" shipping with Drupal. In this case I suggest you implement hook_form_FORM_ID_alter() in a form similar to this one:

mymodule_form_user_login_block_alter(&$form, $form_state) {
  $form[ pass ][ #size ] = 43;
}

Hope this helps! :)

I think in this case you have to go to the your html ( or tpl), not your css file to edit it.

One quick way is to search the relevant string (i.e., size="43" name="name" etc) in order to find the correct part.

Here is the way to theme a user login form in drupal 6 with the preprocess function in a template file and not in a module.

in template.php put this code:

function yourThemename_preprocess_user_login(&$variables) {
  $variables[ form ][ name ][ #size ] = 15;
  $variables[ form ][ pass ][ #size ] = 15;
  $variables[ rendered ] = drupal_render($variables[ form ]);
}

create a new file user-login.tpl.php (if it s not already there) and just paste this:

<?php print $rendered; // this variable is defined in the preprocess function user_login and print the login form ?>

Don t forget to clear theme cache or system cache in the performance settings.