How can I stop windows from being resized on fast-user-switch?
It's a known bug of the 10.9.3 update that has been reported last May. Sadly, it has yet to be fixed.

Possible workarounds:


Hide your windows before fast user switching. The app Notes is immune to the bug, so you can switch to that app and hide all other windows from there.
Use the app stay (cost $14.99, not checked)


Another workaround that works for some people (but not for me) is to kill the dock before fast user switching. Can even be automated following these instructions.