Quote Originally Posted by perfumbg View Post
No need for help I just found it, if somebody needs the same thing here is what you should do:

In includes/clases/OnePageCheckout.php around line 1204 (in my case)

You can simply remove $field_len

from here zen_draw_input_field($field_name, $field_value, "$field_len id=\"$field_id\" placeholder=\"$placeholder\" $field_required") . PHP_EOL .

This removes the whole size thing from any input fields, now you can make them responsive with css.
Just be sure to add sufficient comments to identify your changes, as the OnePageCheckout.php class is the "heart" of the OPC's processing and could easily be changed on a subsequent update.