Even when you assign a 'size' IE 6.x for Windows XP gives password and text boxes different lengths because a text 'character' is apparently shorter than a password 'character'. The only way to get the two boxes the same length in IE 6.x WinXP is to use style tags, because then its based on px instead of on 'characters'
Browsers that don't support style sheets will just ignor it, and browsers that do will be assured to get the boxes the same length.