Swans don't have those long marsh-bird legs, do they?
When you say "big" do you mean big in the body.... or huge wingspan? Sometimes birds look enormous when in flight, but after they land and fold their wings in the mass shrinks alot.
If not a crane/egret/flamingo I cant imagine what else it could have been.
We had several pelicans early (October) in the Gravi, but they seem to be gone now, prolly be back in Feb. They fly eith their necks tucked in though, and dont have those stilt legs. Hmmm