/wp-admin/wp-login.php’s size is defined in /wp-admin/css/login.css
towards the bottom, ctrl+f search for #user_login
#user_pass,#user_login,#user_email{font-size:24px;width:97%;padding:3px;margin-top:2px;margin-right:6px;margin-bottom:16px;border:1px solid #e5e5e5;background:#fbfbfb;}
change “font-size:24px” to “font-size:64px” or whatever your into.