Size of an opening to allow for runoff
What type of soil are you dealing with?
Permeable soil will favor water infiltration, from which you have to deduce the volume. To answer your query, you need to know the types of surfaces, the existing rainwater collection network (is it partially usable?), slopes, and existing drains.
Otherwise, you would have to open up several outlets of 10 mm in diameter along the border of the property.