/etc/signon-ui/webkit-options.d/login.live.com.conf is in account-plugin-windows-live 0.11+14.04.20140409.1-0ubuntu1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 | ViewportWidth = 420
ViewportHeight = 320
UsernameField = input[name="login"]
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