[skellum]

Good afternoon Edward.
I seem to keep buying cars that need work on the brakes, so have now met a few seized unions. Your brake union should need an 11mm flare spanner, and if the 11mm feels 'too big' then as rrobson says a combination of rust and possibly previous heavy tightening has started to mangle them.
If you really need to change the flexis then clean the unions, soak in plusgas a few times and let soak. If they won't undo with a proper flare spanner add heat, but this needs a bit of care so you don't cook the flexi by heat transmission through the nut. (edit- just reread that; if you're definitely changing the flexi then just whack the heat on!)
Sometimes they just won't come.The front unions in particular sitting in the wheel arch exposed to all sorts of salt and muck. Plan B is to cut the rigid brake pipe, and knock a tight socket on. Gives more leverage, but I still had to use an Irwin socket for rounded nuts on one of mine. It means a new brake pipe of course- hence the advice to leave the flexi if you can.
Good luck.