I have been using KeePass to generate and store these passwords; its default setting is for twenty-character passwords with upper, lower & digits. These are pretty secure, but they are also very wearisome to type. KeePass allows the user to copy the username and password to the clipboard, which in most cases makes using it convenient, if a bit insecure (other applications might peek at the clipboard). However even with VirtualBox guest additions installed and working, the Microsoft OSes I have been setting up do not allow pasting passwords into the login/unlock screens. Typing them in got old fast.