I personally don't feel qualified to give much advice. I just instinctually went for this and figured I'd observe and learn as I go.
There are various places online for calculating how big a pond per the catchment area up hill. Might need to spend some time searching terms such as "pond catchment calculator" etc...
In my scenario, I have my spillways exiting back onto my property to ensure no water dumps onto the neighbor's property.
The spillways lead right to many micro-swales all over my property so I'm not concerned of catastrophes.
However, with my limited experience, I'd suggest that you pay very close attention to where you're pond water runoff would occur.
Make great efforts to direct your spill way in a manor that will not leak water on their property. Pay attention to the contour of the land all around (especially downhill of) your pond site.
After the spillway, create a swale or drainage ditch tipped slightly towards your property if necessary as a redundant backup safety feature.
All of this of course depends on the site. You have to be aware of how all the land lays. I did not guess on this. I used an A-Frame level to survey the land prior to even thinking of digging.
Also, add extra clay to the pond were it borders the property boundary.