What you do is set a centre point, and then a size. The worldborder will be moved / resized dependent on what you set. What you can't do is have a non-square area, so if you want a wall 3,000,000 away from your centre at one edge, it will be 3,000,000 away from the centre point on all 4 edges.
If you want to stop players getting to a specific area of the map I suggest using barrier blocks. They're invisible and unbreakable in survival.