Go into the properties of the particle or click inside cascade on an empty place should give you all the options for the particle system. The one you need is called “Bounds”, Enable “Use Fixed Relative Bounding Box” and then input the desired size.
This is so that particles not seen by the camera wont be calculated thus freeing some “ms” from the scene pass, Useful for optimization remember that though and don’t extend the bounds to oblivion unless it’s an important effect that must always be seen.