Hello,

You can assign a specific width to the text boxes available in the file:

CMSWebParts\Membership\Logon\LogonForm.ascx

This file is basically the given log on form web part. You can add various style classes to other controls on the page so you can style them in your stylesheet according to your needs. Additionally I would recommend you to clone the web part before you make any changes and use the copy of the web part instead of the default one, since it can be changed during hotfix /upgrade package application and you can loose your changes.

Best regards,
Boris Pocatko