The distance between two points is basically just the Pythagorean theorem: x^2 + y^2 + z^2. If you have extra dimensions, you just add more terms to this. Now, if you know that e.g. x is very large then that puts a lower bound on the distance:
Distance^2 = x^2 + y^2 + z^2 + ... > x^2.
In other words, if x is large there's no way that the two particles are still somehow close together.