I'd say that the more space they have the better (and the less chance of them rooting up your trees

)
We give at least a third of an acre to get 2 weaners to pork weight and needed it last year because the weather was so wet ! (The wetter it is, the quicker they turn it to mush and the longer it takes to recover

)
So ideally, 1000 square meters would be about right for two weaners IMHO.
As to them rooting up trees, providing the trees are well established (over 10 years old) they shouldn't
really root them up, but if the ground is wet and all vegitation in their paddock is gone - they might be more interested in the trees.
HTH
Karen