A few things to consider if you have too much watershed...

1.) The emergency overflow will get more use hence more erosion.

2.) Fish get washed out more often.

3.) A large rain event can change the temperature (or other qualities)of the pond water too quickly and shock the fish causing a fish kill.

4.) The more water that flows into the pond can cause it to fill up with muck sooner.

I don't guess these are deal breakers. My pond (on the property when I bought) has excessive watershed and did OK before I renovated it. Dam was still intact, void of fish for whatever reason, and had 8 feet of muck in the bottom. I still had it dug out and hope to deal with the excessive watershed problem.

7 to 8 acres sounds about right for an acre pond depending on your yearly rainfall, soil absorption (pond leakage), evaporation, etc.

Fish on!,
Noel