You can get the installer for Windows here: WUBI installer for Ubuntu
Check the attached image for the Wubi screen. the password and username here are meant for your Ubuntu installation, not Windows. Even 5 GB is enough for a basic installation of Ubuntu, however, you will need more as you install software packages and user files under the Ubuntu drive.
Ubuntu definitely supports installation without a password, however, being a linux environment you will be in trouble soon when you perform administrative tasks. Check here: Ubuntu without password Even if you enable the password for Ubuntu account, Ubuntu offers you an option to automatically login at startup. So it is highly recommended to use password for Ubuntu.